Hello, I’ve created a flow but it doesn't seem to be sending emails.

Subscribers sign up via a form and they will then go onto my master list (P&G Master list). They might be on the Master List already, but I want the flow to trigger when anyone (whether they are a previous subscriber or not) signs up to that new form.

I've set the flow to trigger when someone enters a segment (the segment is created by anyone signing up to that new form). But it still does not seem to work - the test emails do not get added to the segment, and the flow does not activate either. Can anyone advise?