Kohler or sterling shower no hot water

My shower recently stopped getting hot. I have tried to adjust the hot water from the valve or handle with no luck. I have an older type kohler single handle shower. I replaced the mixing valve and still no hot water. I took the shower head off and I have hot water coming out but as soon as I put it back on the hot water stops

 

Where is the hot/cold water shut off for the shower? Do you know if your valve body has stops or a shut off perhaps in the basement? With the water off and the mixing valve removed, turn on the hot water to see of water flows to the valve body.

 

Did this confuse anyone else?

 

I think it is an old valve seat or other obstruction closes off the water flow. Without a shower head there is little resistance and the obstruction moves, slow up the water flow and the obstruction closes.

Another is the balancing spool inside the mixing valve but it was replaced.

 

This is the mixer valve for the handle

 

So what would be the best way to clear the obstruction

 

I am confused to lol

 

Look for this with the gray cartridge removed. If you see black plastic this needs to be changed. If you see only brass additional troubleshooting will need to be done.