My hot water goes off in front shower any time hot water is used anywhere else in house. It is a single handle delta. The same bathroom sink takes several minutes to get hot water to it, but it's a double handle faucet. There is a tub in that bathroom with a single handle faucet and the kitchen is a single handle faucet as well as the back bathroom shower.
Is this the cartridge in the front shower? Or could it be the mixing valve in the wall?