Sorry if I'm misunderstanding, but it sounds to me like it's doing what you wanted it to do?
You want to receive the initial down device alert.
And you want a 2nd alert after device has been down for 15 minutes.
Or is it that you only want to receive the email alert after the device has been down for 15 minutes?
If it's the latter, then you should have 2 separate device down triggers.
1) Device Down w/ condition Minutes Down Threshold = 1 min, no email
2) Device Down w/ condition Minutes Down Threshold = 15 min, yes email