Hard to set good Temperature shower

This is the old one, I replaced it with a new one, (waltec) Issue with both is its really hard to set a good temperature out of the shower. I turn the hot on, and barely turn the cold on (1/16 of a turn) to get a good temp. Thing is the slightest adjustment to the cold will change the temp quite a bit. What could be the issue

 

Shower head is a water pik hand held wand

 

solutions@qwaltec.com
4807851234.
Open cold time a 5 gallon bucket then hot see what the difference is, close to same pull showerhead and do the same
then contact them

 

Its only when the shower is used. Ive heard it might be the restrictor in the shower head

 

There's only one pipe to the shower riser...the head doesn't know hot from cold as it's mixed in the pipe before it.

What is the volume out of the shower if you just go cold versus hot alone?

 

I tried adjusting the volume via shut off valves. I'm going try taking water restrictor out.

 

Just unscrew the shower head. Try all cold, check the volume. Try all hot, check the volume. Then, try it with both full on. Note the differences, and get back.