You say that you're "losing pressure." I presume there is some pressure at first and it comes and goes.
Some possibilities are:
# Air in the pump. When you connect the water to the washer, be sure there is water flowing to it (no kinks in the hose, water is turned on). Without starting the washer, hold down the trigger on the wand until water sprays out in a steady stream (no air spurts!). Only then should the washer be started.
# Occasionally, there might be air in the supply hose which, when it reaches the pump, may cause a loss of pressure. If this lasts for more than 10-15 seconds, I would shut off the pump and bleed more air as above.
# It may be something in the gun assembly. Here's a link to a parts supplier that may have some useful info for you ; https://www.ereplacementparts.com/mitm-cba35040mvb-industrial-cold-water-parts-c-163358_163359_163565.html
# If none of these steps work, and you have ever run the washer without water or you have stored the the washer in an unheated area where the temperature falls much below freezing, the pump may have sustained damage. But if either of these were the case, I would suspect that you would have no pressure at all.