Rhododendrons: one third of blossoms didn't open

We have two rhododendrons growing on the South side of the house. One plant has about one third of the blossoms that look like they are ready to blossom but have never opened. The rest of the plant has bloomed nornally and the blossoms are near ready to be picked. does anyone have any idea why this is happening and is this a sign of a problem with the plant itself?

Thank you for your thoughts.

 

Could the flower buds have been killed over the winter from cold weather ?

 

Has the plant been prunned or are the branches twiggy and thin. I find that if I don't prune mine once in a while the bloom get smaller and some of the buds die off, to much competion.