Trigger email not always sending when renter agreement is signed

Hello, 
I have a trigger email that should send when the renter agreement is signed. It seems to me that it only sends the trigger email if my guests signs the renter agreement when it is sent upon booking.
A second email requesting the renter agreement to be signed comes 14 days before arrival. When someone signs the renter agreement from that email they do not get the trigger email.
How do I trigger that email to send if they sign the renter agreement with the second notice email?
TIA