I've had this happen a few times - made a trade offer to an AI GM who wanted to think about it, and then a couple days later sent me an email with a list of additional players he'd accept to get the deal done.
But when I add one of those players (obviously the one I'm most willing to part with) after clicking on "discuss trade again" from the email, I still get the "I'm thinking about it" response rather than an acceptance. An example here where I added the first guy listed, Sanchez:
Anyone else have this happen? Generally I've just clicked on "force trade" since I'm accepting their counter, but it seems like a bug that the devs should look into.