The following Everlasting varieties produce blooms only from old wood: Coral, Amethyst, Bride, Green Cloud. Old wood means that the stems that the plant had in late Summer of 2018 or in the Fall of 2018 will develop invisible flower buds at the ends of the stems and these will open in the Spring of 2019. If winter of 2018-2019 was too cold, it may kill the flower buds or the stems completely. In that case, all you get in Spring is new growth that has no flower buds until late Summer 2019 or in the Fall 2019 but they will open in Spring 2020 if they are not zapped again by a harsh winter. So, if the one that hydrangea that never blooms produces flower buds only on old wood, you may need to always winter protect it to get reliable bloomage in the Spring.
But there are other causes/possibilities:
1. Pruning at the wrong time... bigleaf, mountain, aspera and oakleaf hydrangeas develop flower buds for Spring 2019 on the previous Summer/Fall of 2018. The flower buds are invisible and can develop as early as in mid July (over here... but later for you). The flower buds are located near the ends of the stems so if, for example, you were to prune in January 2019 then you would be cutting off the Spring 2019 flower buds and probably would not have any bloomage in 2019. Thus, it is best to prune these hydrangeas after they have bloomed (so you can enjoy the flowers) but before the end of June.
fyi… normally, if a hydrangea is planted in a spot where it can attain its estimated size at maturity, there is no need to prune... except to remove dead wood in late May or to remove crossing branches (do it at any time).
There are new varieties of bigleaf and mountain hydrangeas that will bloom twice a year. They are called "rebloomers" commercially. With these "rebloomers", the stems from last year produce flower buds in late Summer to Fall 2018. These flower buds then open in the Spring 2019. But later on, these hydrangeas bloom again. These remontant hydrangeas also bloom from new stems that begin to grow in Spring 2019. As soon as these new stems get tall and old enough in the Summer, they develop flower buds and bloom right a way.
2. Lack of moisture - hydrangeas go dormant during winter but that does not mean that one can stop watering them if it is dry. Depending on local rains and soil types, you can reduce the amount of water and-or the number of waterings per week in the Fall but if winter is dry, give them some water if the soil has not frozen. I try to deeply water when temperatures are about to tank well below freezing (like the teens or less).
In the growing season, aim to give them evenly moist soil with no periods of dry soil then moist soil then dry soil again, etc etc etc. Inconsistent soil moisture levels can make the plant abort the flower buds. I planted one hydrangea in late Fall and killed it over the winter when I turned off the drip irrigation when temperatures dropped below freezing and stayed below freezing for several days. I finally noticed a camellia whose leaves were browning and discovered that the soil was all dry and the sprinkler was turned off. The camellia was fine afterwards but the new hydrangea did not come back in the Spring.
3. Pests that can eat the flower buds: deer, squirrels, bunnies
4. Dense shade - some sunlight is needed to produce bloomage but bigleaf hydrangea can produce blooms in dappled sun and in as little as 1-2 hours of morning sun. Bigleaf hydrangea does best in morning sun (up to 11am-12pm ish), dappled sun and "bright shade". Give them afternoon shade and evening shade, especially in the hot Summer months.
5. Too cold - If temperatures during winter are too cold or if the soil dries out when temperatures dive well below freezing, either the flower buds can get zapped or the stems and buds can get zapped. When the stems get killed, the old stems do not leaf out and all you see is dead sticks and lots of new stems taking their place. A hydrangea that blooms only once a year will then not bloom in year one.
In cold climates (zones 6 or less), bigleaf hydrangeas that bloom twice are preferred. As well as bloom hardy ones like hydrangea arborescens (aka, Annabelle) and hydrangea paniculatas (aka, Pee Gee hydrangeas).
Hydrangeas that bloom only once could loose their stems in your zone if it gets too cold so, some people give those hydrangeas winter protection to protect the stems from drying winds and cold temperatures. Even down here, I get spells in January where temps go down to the teens in some years so I (a) water before the temps go down and-or (2) winter protect the bigleaf and mountain hydrangeas.
If you have a rebloomer variety, you stand a better chance of getting blooms at some point. If winter were to be mild, you would get blooms in Spring. Then you would get more blooms in mid-to-late Summer from the new growth/stems.
With rebloomers, you can choose not to mess around with winter protection. The stems die in a harsh winter, of course. The cost comes at the expense of Spring bloomage. But if all goes well, the rebloomer hydrangea will produce flowers from the new stems late in the Summer or thereabouts.
If you use winter protection (add it when the plant goes dormant in the Fall), make sure that you remove the winter protection around 2 weeks after your avge date of last frost and not before. A late frost might kill flower buds if the plant has already leafed out.
6. Too much nitrogen - once hydrangeas becomes established, they do not require much fertilizer (exceptions: sandy soils and potted plants). I let my established, bigleaf hydrangeas feed off the decomposing mulch only. On the first 1-3 years, I give them a single, annual application of a slow-release, general purpose fertilizer. You can use an organic meal like 1/2 to 1 cup of cottonseed meal in Spring or a chemical fertilizer with a NPK Ratio of around 10-10-10 per label directions. If you use too much fertilizer or if you use fertilizers with a large amount of nitrogen too often, you end up getting lots of nice, dark, green leaves with no/few blooms.
