Hi, 

It could be overwintering, pruning time, age of the plants, winter hardiness etc.  

You can refer to our pruning hydrangea page to understand the proper pruning time for Bigleaf or Macrophylla hydrangea. This Cityline series is not a reblooming hydrangea and they bloom on old wood, so you want to make sure to prune only after it blooms and only if it is needed, which it shouldn't be due to its compact habit. 

Most macrophylla species hydrangea bloom on old wood unless it is an endless summer or re-blooming hydrangea it will bloom on old and new wood. This means that they should only be pruned right after they bloom so the stems can set flower buds for the following year. 

Some winters in Maryland are too cold for these hydrangeas causing branch or tip die back. Additionally with these being in containers, they will have to be protected from the soil freezing through to the roots. 

It looks like the ones along the deck railing should be ok if they are left to grow larger and watered regularly. They would prefer to be in the ground, in a morning sun and afternoon shade location though. 

The hydrangeas that are in tree form look like they aren't hardy for our area. Maybe they were florist grown to be temporary plants as opposed to landscape plants. The tips on those are likely to freeze more often than a hydrangea shrub planted in the ground. You could try to protect them in the winter by moving them to a sheltered location where they can still receive rain water. Hydrangea are much more prone to drying out than to be over watered. If they get dry in the winter and then some low temperatures froze the soil, they would likely die. 

Let us know if you have further questions. 

Emily

you're welcome