Answer: Based on your description I am not able to tell you if they are failing to set flower buds because they are in too shady a site or if they are perhaps losing flower buds during the winter due to bud blast for example. These rhododendrons do need a certain amount of direct sun to bloom well -- in northern areas such as yours with a cooler and short growing season they will need more direct sun than they would in a warmer more southern climate. Generally speaking a half day of sun is recommended. I would have suggested a site with full sun all morning or very bright dappled light all day rather than the limited noontime sun. On the other hand if they are growing well and not overly leggy to indicate that they need more light (their normal form is very tight and compact), that may not be a problem.
Flower buds do look different, they are fatter and larger. They can become damaged during the winter and fail to open especially if there is excessive cold or wind exposure, or if they begin to open in the spring and are then hit by late cold. Also, it is possible for the plants to survive excessive cold when the flower buds do not. Yaku rhododendrons are considered hardy into zone 5, and your zip code places you in the coldest part of that zone or zone 5A. Depending on your microclimate, it may actually be as cold as zone 4. This could affect the winter survival of the flower buds. If you think the buds formed but failed, you can try providing a winter windbreak or protective screen made of burlap. You might also want to consult with your local county extension and/or professional nurseryman and see if they can help you troubleshoot based on a more detailed understanding of the planting site and the local growing conditions.
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