A supplier invoice needs to be approved so I put the approver's name as follower. 

Once the invoice is ready, I save it and but the follower is never informed. 

For testing purposes I have even informed him via email that he was being added as a follower and that happened (just to check that the email system was finding no issues with his email address).

We have modified the workflow to move the invoice to Waiting Approval which shows perfectly OK as the corresponding status and the same is reflected in the chatter. 

So, in a nutshell, what may I be making wrong that the follower is not being notified of the change in status?

Thank you in advance,

Luis