I´d like to know the answer to that as well as I have the same problem...so now we will wait until somebody with knowledge of Rhododendrons helps us.

cant say for sure but i had this problem a few years ago i put i down to the very wet weather damaging the outer bud petels and sort of sticking the whole bud in a sticky mess..as i said thats what i thought had caused it but if any brighter spark knows for sure let us know

Are you sure they are flower buds and not new foliage forming. The growth can and does look similar.
How long have you had the plants? I've found in the past they can take a break from flowering.
I'm also wondering that if they are new leaves rather than flowers, are you pruning/dead heading. I took the notion to dead head a couple of Rhododendrons a few years back and inadvertently caused the plants to produce very few flowers the following year.
There are others on site with far more knowledge than me so I'd be interested to know what they think.

Hi, welcome to GoY, it sounds like Rhododendron bud blast, believed to be caused by the rhododendron leaf hopper, it's a fungal disease, which enters the plant when the adult leaf hopper lays eggs in late summer in next years flower buds, there are several treatments, one of which is bayer sprayay greenfly killer, just follow the instructions on the package, Derek.