Delta shower valve has no pressure

So little over a week ago I did a bathroom remodel and put in a delta tub/shower valve and it had poor pressure so I got a new cartridge and it seemed to help a bit but not much and still poor pressure. My boss tells me it might be a manufacturing defect and to replace the valve body itself when I come back for trim because it was already a late day. Ok so I come back for trim today and I decide to try and take the test cap for the new valve and try the pressure without a cartridge in it and now it comes out the spout just fine. I think hmmm maybe I actually had 2 bad cartridges so I go get a third one but still poor pressure so then I replace the hole valve and ??? Still poor pressure. I call up our full time service guy and ask if he has seen such a thing before. He says no and comes over to look at it and if just as confused about it as me. I say maybe delta had a whole bad batch and he finds a cartridge he has had for a couple of months and still poor pressure. Does anyone know what it could be?
Info: 6th floor condo. All copper line exept for the last 2' which is 1/2" pex. All other faucets has good pressure. 4 cartridges and 2 valves with bad pressure but good pressure without a cartridge in.

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