1. Amperage supplied to pump is short of the motors requirement.
Check that the amperage is sufficient at the circuit box.  Motor damage will eventually happen if there is insufficient current to the motor over extended periods of time.

2. Pump is cold.  The pour point of the oil is roughly 10-15 Degrees F.
If you are near this temperature Make sure the pump oil sump is heated.  Move the pump to a "warm room."  If your ambient temperature where the pump is, is around this number in the winter the pump will have trouble starting because the oil is "froze".  Because this condition creates a situation where the pump is inadequately lubricated (until the oil is fluid in the pump) then it is possible to accelerate wear of the pump if this situation persists.

3. The pump is getting contamination from water or acidic components which is causing rust or scale formation overnight that then has to be cleared for the pump to operate.  This issue should be dealt with if it is the case, because eventually the pump will stop working at all.

4. When pump is shut down the inlet of the pump is not vented to atmospheric pressure and oil has backed up into pump inlet causing the pump to temporarily lock.  A review of pump shut down procedure posted on this web site should help rectify this problem once it is understood.  The vacuum pump is not a positive displacement pump ie... it is not designed to pump oil or liquids.  The inlet needs to be relived as part of the shutdown procedure.