Drought is a common reason why this happens. Once the buds have dropped off that's it for this year. You need to make sure the bushes get enough water in summer to form buds for the next year, and during the rest of the year to keep them on the bush. Otherwise they drop off, just as you've seen.

It may also be Rhododendron bud Blast which is a fungal disease the buds will be dark brown covered with hair like growths and they will be firm to touch. They should be picked off and destroyed. Other reasons are very cold frosty weather and as said lack of water in the summer months.