33alfa makes an excellent point. Just to make sure everyone understands the problem....
A vacuum source (either venturi or pump) will move air. The regulator is simply a relief valve.
If the instrument filter is dirty, then the vacuum will appear to increase so slowly it may be unnoticeable. Although sufficient vacuum is created (due to the restrictions caused by the dirty filter)... insufficient air is actually moving (flow) to spin the gyro at rated rpm.
Same thing can occur due to a collapsed liner in an old vacuum hose. The collapsed liner will cause a restriction which will be viewed by the vacuum gauge as increased vacuum, but will be viewed by the gyro as insufficient air-flow.
(It's like buying a pressure-washer. Just because the pressure washer delivers 2,000 psi of pressure will not guarantee it will wash dirt off a truck. It must deliver that 2,000 psi along with about 2 gallons per minute or more, of water. Instead of one teaspoon of water at 2,000 lbs.)
Another analogy would be having 14 volts of electricity available to the starter motor. But only about 1 Amp of current is available at that 14 volts. The engine is not gonna turn over.
Unfortunately we don't have any method to determine flow in our gyro systems, but we can inspect hoses and keep clean filters installed. (Replace filters annually. Most filter have excess capacity and are not a problem even if not replaced regularly. Also, most gyro systems have excess capacity within their plumbing design, and that's also rarely a problem. But 33alfa makes a technically correct point which should be taken care of adequately with regular inspection/maintenance.)
Would it surprise anyone to be informed that our gyros and vacuum systems should be overhauled every 10 years whether they need it or not?
(When was yours last overhauled?)