Little to know hot water pressure in upstairs shower after plumber visit.

Edit: The title is supposed to be "no" not "know"... Didn't check autocorrect; sorry.

Hello, this is my first time posting, so sorry if I'm missing something, but I will gladly provide whatever information I need.

I had plumbers come out to install a vanity/sink and toilet in the basement bathroom. When they finished, the toilet and sink were functional, but the upstairs shower had very little hot water coming out when I turned it on. The cold water came out just fine.

The plumbers came back, and the water pressure was slightly better at the time they finished, but was gone again about five minutes after they left. They came back twice, yielding the same results.

Their suspicion had to do with me having galvanized pipes, which they said corrode easily and tend to cause this situation. The issue for me was that the water pressure was just fine before today, and when I mentioned that, they said something may have knocked loose and then got stuck again in the pipes.

Is there anything I can attempt in order to bring back the hot water pressure in the upstairs shower? Again, the hot water pressure is just fine in all other faucets in the house.

Thanks for any help.