It can be a bunch of stuff
1) nipple is plugged where the rubber air tube connects
2) leaking rubber air tube
3)lazy air pressure switch, stiff, or hole in diaphragm.
4) slow turning venter motor
5) dirty venter motor wheel
6) heat exchanger crack or obstruction
7)blocked vent.

I usually hook up a manometer and see what the signal is compared to the switch setting and work from there.