Workflow Rule not running

Workflow Rule not running

Hello,
We have a workflow rule that every day at 6am creates invoices from [monthly] contracts, based on the contract start date.

On Jun. 30, the rule ran, invoices were created, and we see in the Audit Log, all the actions
On Jul. 2, the rule ran, invoices were created, and we see in the Audit Log, all the actions

BUT On Jul. 1, we see NO invoices created, NO lines in the Audit Log either. In fact we don't see any logs in the Audit, until 8:08 am on that day, which made us thought, there was an outage of some sort.
To try and recover the missing invoices (not generated on Jul. 1), I created a new Wokflow, which the exact same conditions, but instead of running DAILY at 6am, I set it to look at start date + 1 (basically "yesterday") and run ONCE at 2:30 pm (this was done shortly after 2pm, with enough time for the rule to run) - at 2:30 we saw no logs in the audit, and not invoices were created.
Since the [regular] 6am run, always generates logs at 7:08 (1 hour an 8 minutes later), I waited until 3:38pm, but still no invoices and not logs.

Important to mention, that today, we still don't have any invoices, but we do have invoices created for Jul. 3 as expected from the daily run of the original rule.


Is there a way to see when things DON'T work, and WHY?

Thanks,
M.