You did not mention what varieties of mopheads you have so I assume they are not re-bloomers. The last two growing seasons, mopheads have had a rough time blooming due (a) to extremely cold winters that killed either flower buds and-or the stems or (b) due to wild temp swings that made the plants break dormancy and theeeen the flower buds got killed when a cold snap showed up. This becomes more a problem for mopheads that bloom only on old wood. For reliable mophead blooming in PA, I suggest applying winter protection or using re-blooming mopheads like the Together & Ever Series, the Let's Dance Series, etc. The re-bloomers will probably loose the Spring flower buds when winter is bad but they will re-bloom again later on when your warm weather arrives.

Pruning is neither the problem or solution most likely but rather weather issues are. Invisible flower buds developed here in mid-July and will open in Spring 2016. In PA, the probably developed somewhere between late July to August. In order to not loose bloomage from those flower buds if you still decide to prune for some reason, I would prune after they have bloomed next season but before the end of June. Otherwise, if you prune after they have developed the flower buds (i.e., now), you would be pruning off Spring 2016's blooms.

You'll need to provide winter protection to guarantee more blooms. This basically involves covering or shrouding the shrub with any amount natural or sinthetic insulating materials during dormancy (December - late April). You can web search here or Google to see various methods people have used.