The unloader often goes bad if you allow it to run along time without having the trigger pulled, home units can ruin a unloader in 5 minutes, since the water in the pump gets hotter and hotter from the pump.
Also if there is water leaks in the high pressure hose the unloader will "cycle" when the trigger is not pulled, often many times a minute, this just wears the unloader out.
Often it can be merely an adjustment of the unloader spring.
other possible problems
1. if you have a back flow value on the faucet, with pressure washers, esp if the water flow is limited, it sucks air, causing the pump to cavitate and lose pressure
2. most have a relief pressure disk as a safety, often if left in bypass (mentioned above) the disk will rupture causing lower pressure
3. very ( and I mean very often) if they have a upstream soap injector
( this is on the water house side) if it is open a little, you will be sucking air into the pump cauing a air bubble and losing pressure
4. Trigger guns, most last a year or so, on home machines, those used daily will replace the trigger gun about every 3 months, and they can jam with the pressure, and you turning it off would release the pressue
5. You are using the wrong size nozzle, or the nozzle is partly clogged.
Nozzles have sizes, this size is based on water volume and pressure.
This is a size on most washers from a 3.0 to a 7.0 but there are smaller and larger. Then of course you have degree of wash, from 0, 15, 25 and 40 ( on quick connect nozzles, the degree is shown by color, red, yellow, green and white, but the colors are just the angle of spray, not their size,
These are several other things that can cause this.