I've got an older, but not heavily used, K machine, 2400psi, 5hp engine.
It can be the lack of input pressure/volume, I donno, as you mention?
They also develop a problem where the valve sticks.
I found simple directions on the net on how to solve that - take off the silver cap, take a tiny screw or 3 out, run it so the valve pops out (as they say, the toughest part of the whole process is finding the valve again when it flies away...) putting some grease on it, and re-assembling.
Used to have to take the nozzle off, run it so water streamed out, then put the nozzle back on, and it was good for one spraying - let up on the trigger, and it would have no pressure after that. Take nozzle off, let it stream, and good to go for one use...
This sticky valve was the problem, took me longer to type this than to fix it. Search internet for diagrams and simple instructions if this is your problem. The one pump style of theirs is really bad at this.