I have had some that do that. There were no obvious problems "above the ground" that I could see so I decided that they were busy mostly rebuilding their root system from the time the roots got cut off so the plant cold fit into the pot. They took off in 2-3 years but never explained what happened to me. :o)

In the mean time, I would make sure it has 3-4" of mulch year around and I would clear some of the weeds/grass as they disturb the small roots in the top 4" which hydrangeas use to absorb water. Make sure that you provide them with soil that is evenly moist (no periods of dry, wet, dry, wet, etc) and see if they take off eventually. During the growing season, you can fertilize in the Spring once (say a cup of composted manure, organic compost or cottonseed meal) and then give them coffee grounds, liquid seaweed or liquid fish until the end of June so they go dormant at the proper time in the Fall and do not stay in grow mode. Basically, jut give them TLC until they grow faster, which removing the weeds/grass and preventing them from returning will probably help some.

Also, since in your zone 5 the stems may tend to dry out to the ground, all of the growth restarts from scratch at the crown every year. So perhaps try winter protection... it may help the stems get bigger if you do not loose them every winter (the stems from last prior growing seasons will remain and then they get yet bigger on current growing seasons... as long as you winter protect).

It matters some what variety you planted since the size yours currently are is the mature size for some, so if you bought a smaller variety, which is what many folks are looking for, that may be it. As per Luis's suggestion, remove all of the surrounding plants within the drip line and mulch well.