SpamAssassin service stops scanning emails until it is restarted.
This can happen if SpamAssassin is overloaded. To fix the problem you can increase the MaxChildren and/or MaxSpare settings in the "C:\ProgramData\JAM Software\spamdService\SpamAssassinServiceController.config". Increasing MaxChildren allows SpamAssassin to use more processes, which allows it to scan more emails in parallel. Increasing MaxSpare allows SpamAssassin to keep more idle processes on reserve to react faster to load peaks.