Some logs stopped to be sent (or to be registered)

Very strange behaviour on the SEP Manager.

I had the sensation that some clients logs are missing on the SEPM (missing events in Monitor/Logs).

The suspect was confirmed by something very strange: if I try to see System - Client Activity of the last 24 hours I see nothing! I have a lot of system in activity in this period of time.
Other strange thing: I have some logs locally on the clients (such as Application Control) but these logs are not present on Monitor/Logs (but standard Tamper protection logs are present).

It seems that, at a certain moment, the logs on the SEPM simply stops to be recorded in the DB (at least some of them).
Everything worked fine till some days ago.

How could I check this and solve this urgent situation?
Suggestions?

Thanks!