Lost power steering functions

The mechanics in the shop are trying to figure out why after changing the pump twice on their own expancies the problem still remains. The is a possibility that my original pump wasn't bad at the first time when they said so. When I did my own simple diagnosis, the pump does send oil through the system. GM recommended ACDelco is all I used. The PS pump gear box does not show any signs of leaks. What else should we be looking at? Could the PS gear box gone bad? Pls advice.
Thanks,
Mo

Thanks for the back up suggestion Ken. I'm also looking into my rack & pinion oil flow pressure if the meant be any clogging in the system & also checking for the right pressure. Will keep all updated with my true findings. Thanks again,
Mo

To all, the answer to my steering being stiff during turning or packing has been resolved. The problem was my Rack & Pinoin high pressure flow line that got clogged and not sending enough pressure flow to the entire system. I have went ahead and replaced it with a new one from Cardone retailer and everything is back to normal. Even though my PS pump did not go bad in the first place as said by the mechanics, we came to agreement in splitting just the cost of the new one installed without paying for that labor, but the cost of my own provided rack & pinion and labor for installation. Lessons learned for future diagnosis will be PS pump pressure check, Rack & Pinion pressure check, before heading to the PS pump gear box. Also pls do not forget to grease up your ball joints. Thanks to all until the next problem happens again to something different. 2 heads are better than 1 and thank you 2carpros.

Mo