Delta monitor 1700

What you probably have is a decrease in flow rather than a decrease in pressure. The flow decrease is probably caused by a flow restrictor in the shower head.

Flow restrictors typically are a small piece of metal or plastic with one or more tiny holes. They were originally just pressed into the opening of the faucet or shower head, and you could pull them out with a hooked wire. If you're lucky, you can still do that. The restrictors are now sometimes permanently built into the device, so you have to _drill_ them out. Good luck with that.