I have experienced the same thing this spring. Two factors come to mind as possible explanations. First, for whatever reasons, there was especially heavy bloom in the spring of 2009. Even with deadheading, many rhododendrons have a tendency to take a year off after heavy bloom. Second - and I'm not sure this applies to your area - last summer was extremely cool and wet. This provided ideal vegetative growing conditions for rhodendons at the expense of bud set. The lack of sun also limited bud set.

The sparse bloom this year is not universal, however. A number of varieties are blooming better than ever before, so my explanations may be completely off.

Did your watering habits change or was your late summer weather more dry last year? Drought in late summer when buds are being set can be another reason for fewer blooms.

The Middle Atlantic region of the US had terrible bloom set on elepidote (large leaf) rhododendrons this year. This year's flower buds were formed last summer. Last summer here was very rainy and dreary, sort of like spring and fall can be in the rhododendron growing areas of the Pacific Northwest. This is a widespread problem that people are seeing for the first time in many decades. Don't worry. Some varieties did well but others that normally bloom their heads off didn't bloom at all.

I was checking my records to see what was different about last summer during the time that the flower buds were setting. Here in Berks County PA, we got LOTS of rain in 2009 and you probably did also.

May: 6 inches
June: 6 inches
July: 4 inches
Aug: 9 inches
Sep: 3 inches

That is 28 inches in 5 months. Normally we get half that.

In comparison, in 2008 we got:

May: 4 inches
June: 1 inches
July: 5 inches
Aug: 3 inches
Sep: 5 inches

That is 18 inches in 5 months with a dry spell in June. Our 1 inch of rain fall in June is about equivalent to the entire summer in the Willamette Valley in Oregon. So perhaps less summer rain is better for bud set.

Even though we had a lot of snow last winter, the temperatures were very mild. So we shouldn't have lost any buds other than to fall blooming and deer. I don't think winter cold was the problem.