new shower head, no hot water

I installed a new shower head. The actual installation was no problem. But then when I went to have my shower this morning, all I got was freezing cold water, no hot.

Aside from not knowing how to fix this, I can't even figure out how it's possible! This is a standard bathtub shower, where you set the temperature & flow using the faucet taps, then flick the switch to send the water to the shower head. The water is going into shower head is from a single nozzle with the hot & cold water already mixed. So how can this be happening?

Oh, and I took the new shower head off and put the old one back on, and the hot water returned. So the problem is definitely with the new shower head.

It's a Delta shower head, massage variety (I don't know the model). It does have a valve for regulating flow, but not temperature. And fiddling with the valve doesn't affect the temperature at all.

Any suggestions? TIA

Tankless water heater?

I'm in a condo (highrise) with central hot water.

But like I say, the water temperature is fine at the taps and when I put the old shower head back on ... so the problem has to be with the new shower head.

Sounds like it's the shower head, but it can't be. A shower head simply allows water to pass through and doesn't regulate hot or cold. Although there are some that have an anti-scald feature, these simply shut the flow off when too hot. The idea of the tankless water heater makes sense because they quite often require a minimum flow in order to turn on. My bet is that it is the new shower head being too restricted.