They do need several hours, say 3 hours, of sun on the average in early summer to set flower buds. The other possibility is that they may be setting buds and squirrels or deer may be eating them.

The healthy leaves may indicate they have been getting fertilizer. Fertilizer makes nice healthy green leaves but suppresses flower bud production. They set flower buds in late spring or early summer for the next season. So pruning after they have started producing flower buds will remove the buds.

Dry conditions in the late spring and early summer will prevent flower bud formation. But don't over water, that can kill them.

They can be easily moved to a more sunny location. Or the shade can be lessened by pruning the branches that are causing the shade. Rhododendrons like what is called "high shade". If lower branches are pruned back, then perhaps enough sun will penetrate. Any changes will have to be done before late spring since that is when flower bud production begins.