Hey @Daniele_Caglio,
Is double opt-in enabled on your Newsletter list that triggers your Welcome Series flow by any chance?
If double opt-in is enabled on your list and the customer has signed up to your Newsletter list but has not confirmed their subscription, the contact would have a profile created within your Klaviyo account and may get captured by segments but would not be added to the list and thus not triggering and being served the Welcome Series flow. With double opt-in enabled, only contacts who have confirmed their subscription would be added to the Newsletter list and then trigger the Welcome Series flow.
I would also recommend taking a look at a similar post from @ezerosi regarding a similar topic which other members have suggested some helpful troubleshooting tips:
-David