cold fronts are still moving through, mine didnt leaf out all at once this year. give them a few more weeks. take a look at the buds, see if they're plump (live) or dried out looking (prob dead).

Could be an indication of a soil pathogen, most likely Verticillium wilt, which can produce partial dieback. Unfortunately with JM's, this is also pretty much a fatal affliction and the remaining healthy portions will succumb eventually (sooner rather than later).

I would agree that it is maybe a bit early to be sure the bare portions are dead - JM's don't always leaf out uniformly. If after a couple of weeks and still no sign of healthy growth, I'd take a sample into your local extension office or qualified nursery for a diagnosis. Be careful with any pruning and be sure to sterilize pruners after making any cuts - VW is very easily transmitted by contaminated tools.