bathtubshowerwater-pressure
I have a tub shower combo where the tub and the shower have separate taps but I believe share the same water lines.
The tub part has tons of pressure but the shower has hardly any.
I've recently fixed the cold water tap on the shower, it was leaking and needed new o rings, this restored a little bit of pressure.
Would a new shower head help? Could it just be a gravity issue?
It sounds like one of your valves is not working correctly. It could be that the handle is slipping or that the valve is clogged. Does it feel like the handle is turning the valve, or is it slipping? If its slipping you can probably just tighten the screw.
As far as replacing it, you might get lucky and find a replacement valve, but in the event you don't, they make remodel plates which allow you to relatively easily replace a 3 handle setup with a 1 handle faucet. The remodel plate covers up the area in the wall/tile you'd need to remove to swap it out; you don't have to tear out the entire wall. If you can get in behind the shower you can probably do it without the remodel plate.
Could be a clump of solder is in the pipe. I had the same problem recently and ran the shower for a while without the shower head. I never found anything but the problem went away. Maybe it was a glob of flux that finally dissolved.
Best Answer
Try replacing/cleaning the shower head extension (the bit between the shower arm and the head), as they sometimes have screens/restrictions that get gunked up.
You'll always have less water volume out of the shower head, than you do from the tub spout. But if the flow out of the head has reduced over time, it's likely due to mineral deposits.