Hot water mystery

Hi,

I redid both bathrooms in the house with brand new Moen-Adler faucets all the way around and a new water heater and did not change up the copper in the walls at all really, just the connectors to the new faucets..

Anyway, when I turn on the shower upstairs to hot I just get luke warm water, but if I turn on the tub and sink at the same time the shower gets hot like it should. For this reason I did not think it was a matter of adjusting the posi-temp in the faucet and that didn't help anyway.

Is this indicitive of anything familiar to any pros out there?

TIA

Steven