Wondering if there's anything I can do here.

I planted two small (8" tall) Paw Paw's in mid-April. They're planted about 10 feet apart in similar conditions. One is a Canada's Best and one is a Taytwo. Everything seemed fine and both broke dormancy without problem. At this point the Taytwo seems to be thriving, it has a couple branches forming with about 8 leaves on each branch. The Canada's Best seems to be struggling, although it started leafing out around May 10. I went on vacation for 10 days shortly after that and when I came back, all the leaves were gone except for one. The squirrels seem to love eating weird things in my yard like Rhubarb and Day Lily's, so I assumed some critter had eaten them. Well, it started to leaf out again a few days ago and this morning when I went to check on it, all the leaves were gone again except for the same single leaf as before. This time, I looked down and saw the leaves right underneath the tree. There was no damage to them, so it looks like the tree just decided to drop them. There is something wrong that the tree is deciding it can't support any more than one leaf, but not sure where to start. I didn't fertilize or do anything special to the soil, drainage is excellent and I have it mulched to retain moisture. Any thoughts to help it along? Should I water more maybe...it's on a slight slope and some larger trees are overhead that shelter it from rain.