Answer: There are many reasons why they might not flower. First of all they may still be adjusting to their growing sites and becoming fully rooted and established; once they are well established and begin to mature they should bloom better. But after three years, if they do not bloom noticeably better this spring, I would look at the growing conditions where they are planted and also your care routine. You would also want to note whether or not they have even set any buds -- the buds should be on the plant now for flowers later this spring.
Rhododendrons tend to grow well on the east side of the house where they receive morning-only sun and as a result the site is bright but also naturally cooler and moister during summer. On the south side in full sun all day they tend to bake in the summer, and also overheat due to reflected sun during the winter. This can stress the plants and also can damage the flower buds. Generally speaking, I would not recommend planting rhododendrons on the south (or west) exposure for these reasons.
Improper pruning reduces blooming. These plants grow their flower buds the year before they open. Pruning in late summer, fall, winter, or early spring will remove flower buds and reduce blooming.
Overfertilizing, especially supplying too much nitrogen, can result in excess vegetative growth at the expense of blooms. Rhododendrons are naturally slow growers and not terribly heavy feeders;they may not need much if any fertilizer if they are growing in average soil with a year round layer of organic mulch.
Excess shade can also cause fewer blooms. So, if the south side of your house is very shady, that may be the cause.
Finally, some rhododendrons are more winter hardy than others. Perhaps you have planted a variety that is root hardy -- so it survives the winters -- but not bud hardy -- so the blooms are lost -- in your area.
While I can't tell you specifically what is happening with your rhododendrons, I hope this helps you begin to troubleshoot. You might also discuss it with your local professional nursery staff and/or county extension and see if they have any ideas. Maybe they will bloom nicely this spring.
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