Kathy: My oldest r. schleppenbachii is 25+ years old and continues to bloom before the leaves and as well as ever.

A couple of possibilities:
Zone 4b is right at the borderline of hardiness. If the last couple of winters have been colder than usual, flower buds may have been killed.

Has the planting site become more densely shaded over time? Some sun is needed - morning sun, not intense late day sun which will damage the leaves - for good bud set.

This has been an unusual year. The cold, wet spring has caused lots of rhododendrons to come into growth later than usual. in many cases this has meant delayed flowering and simultaneous leaf emergence. Not an explanation for prior years, of course.

A soil test might indicate the need for fertilizer.

In any case, unless the plant is badly overgrown, thus depriving portions of any sun, I don't think pruning is very likely to provide a cure.