Messages are getting stuck in queues. Restarting the K2 Service does process the messages, however with a lot of messages being stuck the service would have to be restarted the same amount of times as the amount of stucked messages that there are.
This is a known issue where the workflow scheduled starts: SourceCode.EventBus.Scheduler.ProcessStarter.ProcessStarter
An existing bug is known - where the sync flag on management studio is the wrong way around, so the Async tickbox on Management page on the schedule needs to be unticked.
*Please change the async check so that the process will start async : synchronous:false,
You can confirm with the debugs (the debugs can be obtained via a support ticket) in the K2HostServer_EventBusServer_ logs that the AssemblyTypeName: SourceCode.EventBus.Scheduler.ProcessStarter.ProcessStarter has synchronous:True, this should be synchronous:False
If synchronous:True, then MSMQ will start the process, and wait for a client event or process to finish before returning to process the next MSMQ entry.