AC problem again-- what to do now

[Bienes Raices]

I've posted before about this problem--now it appears to be back.

My old tenants complained repeatedly that the house was getting too hot. It has a Goodman air handler inside and a heat pump outside. The AC guy came twice and couldn't find anything wrong with the unit. I added several hundred dollars worth of insulation to the attic to make sure that wasn't causing the problem. Then the tenants told me at the end of their tenancy they were getting $300/per month electric bills. I called the utility company and discovered they made up that whole story about their billing.

Those tenants moved out a few months ago. To be safe, I had a free energy audit done by the utility company representative. He said the house was in mostly good shape, but recommended adding window film to the slider and more weatherstripping to the front door. I did both of these things while the house was vacant.

The new tenants move in six weeks ago. I just received a code violation in the mail from the county about the lawn height. I called the tenants and told them they needed to start mowing the lawn ASAP. At the end I asked them if there were any problems with the house. He said the house is getting too hot in the daytime. Apparently they said the thermostat at 77 but the room temp still reads as 80 degrees. I've never mentioned my issues with this problem to him before.

This tenant used to work in HVAC. He thinks the return for the air handler is restricted, and it needs a bigger return. Has anyone heard of this problem? I'm wondering why the other AC tech didn't mention this to me last year if this is an issue. Or could it be the refrigerant in the heat pump?

I don't know if I should keep trying to "fix" this problem since I've already done so much and I'm running out of things to fix.

Thanks.