Is your soil acidic? If not the rhododendrons need feeding with sequestrene of iron and lots of ericaceous compost forked in around them.

The winters of 2009-2010 and 2010-2011 may have also killed the flower buds for the following blooming seasons. Are there buds showing on the plants this year? Have you gotten in the habit of pruning the branch tips each winter? The flower buds for next spring form the previous late summer or fall.