SylvanLMP
A lot of thoughtful plumbers when they install a showerbody they use an anti scald type that works on pressure or temperature Or both.
Now if this is the case some times these pressure balancing valves need to be fine tuned.
You could also have a lot of debris build up inside your H/W tank that restricts the hot water flow like galvanic corrosion from the discharge nipple (connection)
What I would suggest is contact a local Licensed Master Plumber in your area DO NOT allow anyone NOT licensed in your home.
Then have this plumber go through your system explaining the proper care of your heating/ H/W and show you the main shut off valves etc. Good luck in your home.
