A second floor that runs five to ten degrees warmer than the first is one of the most common calls we take in July and August. It is almost never one single fault.
Heat gain through the attic is the usual starting point. Undersized or leaking return ducts are the second. A system that is correctly sized for the whole house can still lose the upstairs if the return path cannot move enough air.
On a service call we measure supply and return temperatures at the registers, check static pressure across the air handler, and look at what is actually happening in the attic before recommending anything.