Things to check.
vacum leaks, actuator, lines etc.
micro switchs in latch assembly, they will stick and fail to signal the pump that the door is now shut.
trunk latch, loose latchs will allow the trunk assist to keep pulling on it, thereby shutting the pump down.
shutting the door on the seatbelt is a surefire way to shut the pump down too.
I've replaced several latchs in these for this problem, make sure the l/f is the only door shutting down.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.