Reduced water pressure to shower

Hi,
About a month ago, we experienced lower water pressure to the upstairs shower. It was still alright for showering. Yesterday, it suddenly went down to a trickle.

All the other water pressure in the house is fine, to include the toilet and sink next to the shower and the other shower head backing this one in the next bathroom.

I do not think it is the head itself, because until yesterday the water was coming out evenly. The head has been in place only about four years; we're on city water that is not hard.

In the head, there is a center button that can get sucked back inside a bit; I don't know what its real function is. It has sucked back in since the pressure went down and now when turning on the shower it happens each time, as if some back pressure is the problem.

Can anyone help?

I'd first try just taking the shower head off the elbow and seeing if there's any crud lodged in there. I haven't the foggiest idea what the little button is that gets "sucked in", unless it's a cutoff valve for shutting off the shower while you're lathering up. If that's the case, that valve may be the problem, since it seems to be cutting things off on its own.

If there's low flow with the shower head off, see if it's any different for full-hot vs full-cold.

There's a 99% chance that the problem is in your shower head. If that's not it, you have a plug somewhere else in the system, which is much more difficult to isolate.

there could be a couple of things here. firstly, if its like a moen pressure balanced system (moentrol (push/pull handle) or Posi-temp(rotate only handle) then it could be a stuck spool. the other thing, that little button in the middle of the head could be like the button on the moen easyclean showerhead. you would tap the button rapidly, and that loosens debris inside. it might be handy if you could check the showerhead for a name, or maybe check the escutcheon plate behind the handle for a name.