The pump should reach 29" as long as there are no leaks in the system. When using a spoil board, cover all unused areas with 3-5mil plastic sheet taped down at the edges and the sides of the spoil board tape sealed around the periphery
The Exhaust Filter may not be seated. sealed, cracked or clogged.
If there is an oil return valve, it may be stuck open or the return line clogged.
Poor vacuum build can also be related to dirty oil or worn internals if no external leaks can be found.
It sounds like some good TLC maintenance would help. Do not use a detergent oils in these pumps. Bosch R530 is standard oil and R570-590 is for high ambient temp environments.