When they've completed the sequence, they're marked as such. I had once this problem and the support fixed it for me but they said something along the lines of that I have to remove and add the subscriber again. For myself I fixed it the way that I write always 1 email more so that the sequence doesn't end. You could also schedule your email to a time where you know that until then you've already written a new email.

Zhenya
Content Writer / Content Marketing Strategist