If you not descale the unit frequently, a thick layer of scale is created on the walls of the boiler.
Let’s say you never did descale the unit for like 2 years, and suddently decide to descale it, the thick scale build will unstuck from the walls forming big chips of scale on the buttom of the boiler, when you opening the steam knob, some chips may get stuck into the pipe/valve, reducing the pressure.
You should open dismantle the compleet valve and maybe keep it on a descale solution for some time to resolve the p problem.