I am almost positive you had a store that just didn't know how to do the promo correctly. I've taken advantage of this deal many times: for ODST, Uncharted 2, Dragon Age, and Left 4 Dead 2.
The issue is that most promo's at Gamestop are already set up to ring in automatically in their computer system; for this promo, there is actually a code they have to manually scan in. It is in on of those little weekly/monthly information folders they have. The point being that they should have the code somewhere behind the counter, they just might not know what it is since they obviously haven't done this promo yet.
At the two I went to they scanned 2 of the games I had off of the list, then the new game...still full price. They called around to some other stores in the area, and couldn't figure it out, so they actually manually adjusted the price of ODST down to $20 for me (looking back on it, they should have manually adjusted the TIV of my 2 games up to $20/ea, because the way they did it actually gave me ODST for $7!)
The next store I went to had the same issue, wouldn't ring automatically; I go in there a lot, so he honored the deal for me anyway. He just adjusted the TIV's of LSW and some crap Wii game (can't remember which) up to $20/ea, giving me Dragon Age for $20 after the 2 trades.
The last store where I got the other couple games had no issue at all; he knew exactly what promo I was talking about, verified my games were on the printed list, whipped out a little scan sheet, and everything adjusted automatically.
If you've looked at the deal on their website there might be an issue with some of the games...there is an ever growing list of addendums below; ie games they've made ineligible for this promotion, so YMMV on getting those to work. That being said, I traded in 3 of the games that ended up being added, so who knows about that.
sorry for the rambling, but hopefully this will help someone...get them to scan the code!