Low pressure in upstairs shower

Hi, I am new to this forum. I did some serching and did not find exactly what I am asking so here goes.
We have two showers, on on first floor and one on second. The first floor works fine but the second has always had low pressure (8 year old house, contractor was un responsive in help).
What can I check before I pay a huge amount to a plumber?
Thanks,
Lee

First, if you have not already done so, remove the shower head and see if you get good flow straight out of the pipe. That would tell you if the shower head was clogged, or is just plain "wimpy" , which most are these days.

You have about 4 PSI less pressure on the second floor ( gravity ) but unless you are already very marginal, that should not make the second floor that much slower.

Do you have the same model of shower valve on both floors?

One thing that can ultimately be done is to remove the shower head, and remove the "innards" of the valve, and use air to blow out any possible debris in the line. Do this if the flow is weak with the head removed.

I checked floe from the pipe, it is not all that steller but I dont have a pressure gauge. I will pick one up and check it.
Thanks