According to Clark W. Griswold, Jr. 73115 dot 1041 at compuserve dot com:

It sounds like either the lower element isn't working or the supply dip tube is
missing. Either way, the installer should be able to confirm if everything is
working correctly.

Or T-stats grossly miss-set, wiring defective/not attached.

Or the heater water supply and feed lines are reversed.

Or 240V elements have been inadvertently connected to 120V.

The inlaws had a situation very similar to this, turned out the 240V
elements were being powered from 120V. Extremely long recovery times,
could only get a shower and a half before it went cold, etc.

Call the installer back and ask they check out each of the possibilities
we suggested here. Shouldn't take them long to double check them all.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.