It sounds like you are describing what happens to hydrangeas stems when they get injured by winter and either the whole stem dries out or parts dry out. And as a result of that, the plant produces less leaves than normal. In such cases, you may also see some new growth (new stems) developing from the crown.

Since some stems may be partially or completely dead, you can try to prune the shrubs a bit after they have bloomed so the remaining stems have about the same height and are not lopsided here and there.

You are probably one of the few hydrangea owners in zone 3, and it's great you got any type to grow there. I wish some of us here could speak from experience in that zone, but I don't think it's likely anyone has it.

To Springwood Gardens, there are lots of people growing hydrangeas in zone 3. Mostly Limelights, Annabelles and PeeGees. None that I have seen that can turn pink or blue (those are in zone 4).

Since my original post, I have read that Limelights prefer full sun. Having planted on the east side of my house, the 3 that aren't doing as well are the farthest from the south corner of the house.

To luis_pr, I think you may be right about the winter damage. I realized that the two that are doing the poorest were subjected to too much water from the roof downspout in the fall, causing extra ice build-up in winter, and too much water again in spring. Guess I'm lucky they are alive. We have moved the downspout, but I'm going to transplant at least these two into more sunlight. Should I wait until after they have bloomed or to fall before transplanting?