Use caulk, or spray foam insulation on the plates of the wall in the attic.
Check the return duct in the basement to see if it is pulling in basement air.
There was a hole in the duct that I taped up that was sucking in Basement air... but the basement air is quite cool. Which I have been finding random AC air leaks along the duct joints, so taping those as I go. That is seemingly making a slight difference in time the AC runs through the day, but we are also in a cooler period at the moment, so the scorcher 100 degree days will tell me the real impact.
If you have leaks in the return duct. The basement being cooler. Prevents the A/C from lowering the latent load. And takes away some of its ability to cool the house. Basically. The better sealed the supply and return is. The better it will cool your house.