I am in the Northeast (Vermont). I have been gardening at a clients house for over a decade. There is a patch of rugosa roses (not sure of the variety), that used to be full and healthy. The patch is slowly dwindling. The still produce flowers, but the actually number of plants is less.
I think I may have pruned them at the wrong time. What is the best time to prune them?
The rule of thumb "prune roses when forsythia blooms" - which means in spring - holds true for rosa rugosa, too.
Rosa rugosa is basically very hard to kill, it can deal with lean soil, harsh and wind conditions and even semi shade. A typical reason if it gets "lazy" is too many old branches. R.r. blooms on new wood, unlike some other roses that bloom on last year's growth. So if you want consistency, remove old canes and, if necessary, a few newer ones to encourage the plant to focus on the new shots. If your hedge is looking really bad and you made sure that no other problem like insects or fungi is the real culprit, consider cutting it down completely, letting it grow anew from the roots.
