I can't recall having this exact issue, but sometimes DL's are tricky.
Try this:
go into the AD domain and create a new AD Distribution group for the DL.
Then go into Unity and activate it how you normally would. Assign a person to be the owner, etc. See if it works this way for a test. Possibly Unity in SA is not creating it correctly, or if you have large AD domain, the replication has not occurred or has an issue.
cheers