Depending on the age of your WH, awhile back there were a bunch of them produced with defective dip tubes. This is a plastic tube in the WH that directs the hot water down towards the bottom of the tank so you don't immediately mix the hot that rose to the top (where the outlet is) with that incoming cold. A batch of those eventually broke up into little pieces, clogging shower heads and aerators on faucets and generally making a mess of things. You may have a failed dip tube. Flushing out the system after replacement would solve the problem. BUT, if your WH is old enough to have this problem, it's probably on its last legs anyway.
All newer and many older shower control valves have a maximum adjustment independent of the handle that controls the maximum hot you can adjust to. If the flow is partially restricted, that adjusment may need to be changed.
What does the crud blocking your showerhead look like? Stick some of it in some vinegar overnight and see if it disolves. If it is something like calcium, it will disolve. If it is plastic from say a dip tube, it won't. If it is sand (unusual in a municipal system), it won't. Anyway, you can get an idea after describing what it looks like and what happens in the acid (vinegar).
The anti-scald functions in a shower valve can get messed up with particles in the water...often replacement is easier, but sometimes you can remove, clean, then reinstall to resolve the problem. First, figure out what it is that is clogging up your showerhead. Note, if you have any galvanized bits of piping, it could be rust flakes as well. Those are often easy to identify by their color. Many would also be attacted to a magnet.
