return air issue? advice needed

5 y.o. new build 1750 sq, ft. with 2 returns. second return straight line into the first; which ties to the unit.

Issue, house is half cold, half warm, uncomfortable. return duct (one half of house) the toilet paper taped to the grill gets sucked up against grill. Can feel the return air under the bedroom and office doors.

Second return duct, paper flutters a little, next to no movement. No return air felt under both bedroom doors. Lennox techs shrug tell me the second duct does nothing. I ask why did they waste my money, no reply. five techs same lennox dealer no help. They adjust damper to push more air out but then the other (good) side suffers and still same results.

Lived in homes from 900 to 2400 sq. ft. and never had this issue. Their reply is, well it's hot in Texas. wow, insulting. I have tinted all windows same results. A/C systems is good with sub-cooling and pressures. Coils cleaned on regular basis.

Still think it's a air flow issue but don't know what next steps to take.

Appreciate ideas from you seasoned guys. Oh, I called an independent, he shrugged, gave no answer why second duct did nothing. Offered to relocate second duct to a different area up in the attic. Gave me a price, shook hands; then ghosted me 10 months ago.

Now what??? we are in our 70s, not made of cash and see how we get treated by many contractors. really sucks!!!