I live in north west Florida, Temps in the mid to high 90s right now, I just installed a new 2 1/2 ton system, the old 2 ton system was just not keeping up. The new one works great but from 12 in the afternoon till dark it will not satisfy the thermostat. Pressures are good and the supply and return air temps are right on. Getting plenty of return air also. I am thinking I might need to reinsulate the attic. My house was built in 1978 with blown in insulation and it also has single paned windows. I am thinking it's a good time to replace windows also. Anyone got any other ideas.

Just a thought,how big is the house? Is a 2 1/2 ton unit big enough?

Did the installer perform a load calc (aka Manual J) to be sure the unit is properly sized?

Improving your thermal envelope is usually a much better long term investment than upgrading HVAC equipment.