Just a random thought ... are you using the pump only on mains power? How does it get on when starting on battery power (mains off) only?
And what's the condition of the battery? Does the pump work OK on mains when the battery has been removed?
BTW, I'm assuming this pump
has a battery! LOL.
OK ... you've said that the pump worked well when you changed the plunger assembly - but I'm wondering if the earlier one was a bit stiff (too stiff for the battery to overcome).
In other words, does the plunger assembly move freely?
