.... and the resultant flow is less than what your engine requires for WOT and the lift pump cant develop the extra vacuum needed to sustain that flow.

Here's how to develop WHEN to change a filter cartridge .... if needing to change before ~1 year 'maintenance' changeout.
Install a NEW (clean) filter cartridge
Install a pressure/vac. gage between the (each) filter and the engine and the lift pump.
Run the engine at WOT and shut down the tank valve ... record the max. vacuum reading --- max. potential lift pump pressure/vacuum.
(now bleed the system and verify the integrity of the lift pump diaphragm !!!).
Change the filters when the system develops ~80% of the vac. developed/established at WOT stall pressure/vac. .... once such filters go beyond 80% of their dirt capacity at max. system pressure the 'close down' of flow is usually 'exponential' (damn quick).
ALL future 'checking and monitoring' with the gages is done with the engine running 'near' WOT.