I'm going to assume that the vaccuum was sized to the house, so it's not under-spec, although poor maintenance might mean that it's underperforming.
I'd suspect one of three things -- a blockage, a poorly performing pump, or a leak in the system.
If it's a blockage, unless it's right near the pumps, you'd have some areas with good pressure, and others past the blockage with poor pressure. (and the pump would be straining to keep up). You might be able to map out where a blockage might be by determining which ports have good suction, and which ones don't.
If it's a poorly performing pump, it's difficult to diagnose, but you might try listening for sounds of a straining motor and/or any odd smells. (I'm not so good in this field, so can't really say much more, a bad seal could cause just as bad as a problem, and I don't know if there's a good way to test for that.).
If it's a leak somewhere, the whole system won't be able to come pressure, so you'll have poor pressure everywhere. There might be additional dropoff at the leak if you were to try to map out which ports have worse suction than others, but as the suction is poor to start with, and some dropoff is standard for a pressuried system, it might be difficult to identify.
You might be able to better pinpoint a leak by intentionally clogging up the line, if it's run as a line with multiple ports off of it, rather than them all branching from one point. You'll need a two-way radio, baby monitor, or something similar. Put the transmitting end near the pump, and carry the receiving end with you. Open up the port, and intentionally block the tube on the side heading towards the pump. If there's no leak in that section, you should hear the pump straining.
Luckily, leaks are unlikely (unless perhaps if you've had an earthquake in that area), because I have no clue how you'd go about actually repairing it without tearing out the walls.
