Nurture not launched

Hi all,

We started 4 new nurtures, and only one of them has not launched at all? No emails have been sent for over 24h after the scheduled start. It was actually a duplication of the other one (with edited email content) - which works perfectly fine. 

Is there a reason this could be happening? All the settings seem to be fine and the list was used before also. 

I would appreciate any advice.

Best,