I think that maybe it's the zone 5b that might be your problem. Most macrophyllas are not bloom hardy in zone 5 without serious winter protection...Check out Hayseedman's thread on Overwintering hydrangeas......yg

I've said it here before, but we had a landscaper redo our front yard about 5 years ago and he put in 5 Glowing Embers in bloom. They bloomed the next year, but hadn't bloomed since. Last fall I decided to cover them with rose cones as they reputedly bloom on old wood and I have a bunny problem. By spring, all the hydrangeas are chewed down to the ground.
As a result of my efforts, I now have TWO blooms total on the four bushes that are still alive. They do all look healthier this year, but I consider them on probation. I planted a young, healthy Annabelle near them to see if she'll grow and thrive. If so, they're GONE next year, no regrets. I have read that they're hardy in zone 6 and south.
I have consistent blooms on All Summer Beauty and Endless Summer looks promising. I pile leaves on them in fall. Don't know if that's considered serious protection.

I had both 'Glowing Embers' and Alpen Gluen' on a seven year probation! They never really performed (lots of winter/spring kill) for me in zone 7a. They have now been banish to the edge of the woodland. They do have nice form and leaf but so do many others that would do well for you. I wish I had replaced mine sooner. Good luck. Katy

I don't think rose cones qualify as serious winter protection, nor do piled leaves. Again, check out Hayseedman's thread.....yg

Yellowgirl, I have checked and printed that thread. Very informative, but there's a limit to what I'm willing to do.
My point was that All Summer Beauty thrives in my zone 5 garden consistently with only leaves blown on for cover. I'm crossing my fingers that Endless Summer will do likewise. That'd be two mopheads that bloom well here with little fuss.