The ants take the food to the queen. If she's got enough food, they won't bring any more.

Fire Ants have been known to 'learn' to avoid certain ant baits. I think it takes more than one application, but they do eventually get the hint. On the up side, it's easy to fool them. In my experience, they seem to only be able to recognize one 'bad food' at a time. So if you have Amdro and Ortho and alternate them, they don't seem to be able to keep up. I don't bait often, only when it gets to the point where I can not sit in a lawn chair without being attacked, but when I do I try one bait the first week and another bait the next. This usually buys me about three months of peace from Fire Ants, though the Sugar Ants tend to have a population boom while the Fire Ant numbers are reduced. But Sugar Ants I can handle. They very rarely bite, even when I accidentally disturb their home. Fire Ants are just plain vicious and are in no mood to share my yard.