I have two bathrooms back to back. one with a tub and shower and the other is a walk in shower. Both have the same shower head. One has super pressure, the other is eh. What do you this causes this?
We had this issue and found it was the gasket inside the one shower head. Same thing - two showers, same shower head, one didn't work right. Once we removed the gasket and reinserted correctly everything was fine. For us, the gasket had been disturbed during our move.
In the US, there is a law that a shower head can only put out a certain amount of water. Depending on the design of the head, there might just be a little filter/stopper piece that can easily be removed from where it screws into the pipe. Some people remove the restriction device to have "more pressure".
If the shower heads were installed by a previous owner, it's possible that one has been modified to put out more water.
Also, some shower heads have an adjustable spray, but the adjustment method might not be obvious. Some rotate, some have a lever, etc. Look at it close and make sure you're not missing anything.
Another possibility is clogging due to lime buildup. You can either remove the shower head and soak it in Lime Away, or bag it and soak it on the pipe.