New AC Unit not cooling the house sufficiently

I had a brand new AC unit installed in July.  Yesterday, the tenant put a repair ticket in, in it she said "ever since this new AC unit was installed, the house will not cool below 77 degrees."  I just got off the phone with my PM and he said the HVAC guy thinks its a dirty air filter (and he plans to change the thermostat as well).   I plan to monitor this with the tenant to make sure it works, but has anyone ever found this issue to be caused by something as simple as a dirty air filter, or is it common to get an AC unit that is a lemon?