Just a hunch here, but on the weak one, check the root ball to see if the top of the root ball is too low and covered with soil on top, where the roots cant breath. Or if the pot drainage is bad. It sounds like when it gets water the roots are drowning, creating leaf drop. Its the same symptoms as drought -- the plant defoliates, then puts on growth. But in your case, the same thing happens again, which makes me think it is poor drainage and maybe planted too low. Whatever it is, the problem is in the roots. Your other one is okay -- the roots are getting air and there is good drainage. Try comparing them, and when the weather turns cool (inducing dormancy) try repotting the weak one in some light soil mix, not potting soil and see if it strengthens up next year. Be sure the top of the root ball is not buried. Good luck!