AMEX does seem to have issues with the Delta credits and how it shows up in their systems. Sometimes the ticket is still valid, but doesn't show up on their "screen," which is a torturous adventure to have the agent go find it.

What I have done to get it off the AMEX system is just book a new ticket. Then after a few days I will just cancel it on the delta website. This will generate an eCredit in the regular way with an expiration date that makes sense and isn't ambiguous.

HOWEVER, once you do this, you can no longer use the credit with AMEX. Only a problem if you need a new ticket where AMEX has a discount, but I generally have enough flights that don't have a discount that I can use up the credit regardless.

(This is all based on using AMEX Platinum not amextravel.com)