Installed new shower head, now no hot water

My husband removed the old plastic shower head in our shower, and replaced it with a nice metal one, which looks great.... but now we've lost our hot water!
It is barely lukewarm, even when the cold tap is completely off.

Both the sink and the bathtub in the same bathroom still have hot water.
The only way to get hot water running in the shower, is to have the bathtub tap turned on and running at the same time.

Any ideas??

Replacing the shower head and the problem you are having should have nothing in common....

To test this remove the shower head and run the shower and see what happens....

Thank you for your reply.

I will try that.

Sorry, I should also have mentioned that both hot and cold taps in the shower were also replaced.
Could the problem be coming from the taps??

Was the valve installed in the correct position...most have an arrow showing the up position...

Beyond that it could be solder jambing the balancing spool or other debris problems...

Some valves have small shutoffs built into the valve...if yours has these be sure they are open...