Yes, PJM requires exceptional drainage. I had the same problem until I planted them on top of about five inches of sand, which in turn are above about 2 inches of gravel. The shrub is also about 3-4 inches above ground. Most of the hybrids with minus var minus Carolinianum in the parentage have similar dtainage requirments.

You might want to try Olga. I have not had the problems with Olga that I have had with PJM. I find this is interesting becuase the parentage is very similar. Olga is often called the pink PJM. IMHO Olga has better color, and blooms right on PJM's heels.

Two other Mezzit introductions are Landmark and Milestone, both of which have done very well for me.

All the above are listed on the ARS.

All rhododendrons are prone to root rot. They need excellent drainage. PJM is about the toughest rhododendron there is. It takes deep shade and blooms. It takes the hardest winters. It keeps its small size.

Before replanting the same area, try a raised bed. It is the best solution for root rot problems.

Here is a link that might be useful: Section on raised beds.