99 Mustang Keyless entry question

Able to program remotes easily enough. Problem is they don't stay programed. Sometimes programing last for a couple days, other times it's less than an hour. Have ruled out the idea of the cars battery causing the problem. (always has a full charge, and able to start the car). Have replaced the battery in the remotes several times. Programs everytime. Just doesn't keep the programing. My daughter works night and needs this to work when she goes out to her car after work. Any ideas would be helpful. Thanks in advance.