Strange... I just tried them again and they are working for me. The one time I had trouble was when I put in a bad code and then every code after that said the same thing (unfortunately, I don't remember what the specific error message was, but I don't think it said Capacity Check). The way I fixed it was refreshed the page, but have read that others have done that (refresh, cleared cookies, etc) and it didn't work for them so most likely I didn't get the same error.
Just a guess, but because of the word "capacity" I wonder if there are only so many discounts they can give under that code and they've reached the limit? TOTALLY just a guess since others have reported that they talked/emailed
Alamo and they said they are aware of the issue. Who knows, I wish I could help....