The most likely cause of your problem is a dried out root ball. Pot grown rhododendrons are very prone to this - there are many examples on this forum. The light peat based medium used by the grower is very hard to rewet once it dries out. The problem is made worse if at planting the roots are not cut, raked with a cultivator or otherwise unmatted.

The best thing to do is to dig up the plant, tease out the roots - don't be afraid to really do this thoroughly - and soak in a bucket until the root mass is wet. Then replant. If you notice signs of wilting over the summer, water at the base of the plant with a slow trickle for 30 to 60 minutes. Don't soak the whole bed. That just makes it harder for the roots to move out of the original medium and into the soil around it. You may have to use the trickle technique again next year, too.