Some of the deadwood may have been alive and, if left un-pruned, it may have leafed out and bloomed. By now, the stems would have been much larger giving you the look of the one on the right. After the end of May, feel free to prune the deadwood.

Since we are not with you to observe details all year around, make sure: they did not have roots circling around the pot when planted; they get afternoon shade (sun can bounce off the wall and make things hotter); the soil is evenly moist (periods of dry-moist-dry cause leaves to dry out and whole stems to die); one side does not get more or less water than the other side (usually due to problems with the hoses); the drip/hose is close to where the roots are (if you water away from the base/crown, the roots will not get much water); the mulch that you use helps prevent water evaporation (not sure how well the rocks will do that); the soil pH is not too alkaline (some types of rocks leech lime; they can also radiate sunlight and thereby overheat nearby plants); they continue to get a fertilizer like composted manure, organic compost, cottonseed meal or a chemical general purpose slow-release fertilizer whose NPK Ratio is about 10-10-10.

I would get the drip/hose closer to the first two and put hardwood mulch/pine needles/etc up to the drip line and then the rocks. I have had some identical mophead hydrangeas behave differently like that before and never quite figured out why one was stunted and the other one prospered.

Thanks for the reply. Oddly the one I didn't prune was the first one on the left. Yet that one has done the worst. I have hardwood mulch so hopefully that helps. Will move the drip hose and get some fertilizer down.

You our said it's safe to prune after may? If so I'll keep that in mind. One consideration is I'm up in Minnesota so perhaps it would be later.

It is safe to prune dead stems on or aftert the end of May.

But regarding pruning, mopheads develop invisible flower buds starting around mid-July where I am and later (August) for those in northern locations like you. These flower buds stay invisible until they open in Spring 2016 and look like broccoli heads. The flower buds usually develop at the end of the stems so be careful pruning stems that are alive on or after mid-July as pruning them now could result in no blooms in Spring 2016 (if your mopheads were the type that re-bloom and you prune them now then they will not have their first flush of flowers either but they will bloom when their second flush is supposed to bloom in your zone).