Low Water Pressure Delta Single Handle Shower Model 602
Is it supplied by old galvanized water pipes? Those obsolete lines will build up scale inside over time, restricting water flow, and will eventually plug up completely if not replaced.
If not, have you turned the water on with the faucet ball assembly completely removed to check the pressure and blow out any floating debris that might be plugging it up when pressurized? Try that.
If you've replaced all of the Delta "innards" with genuine Delta parts, one of the two scenarios above may be the culprit.