This is usually caused when you have the wrong version of the trainer with the game.
originally posted by TTSG
Make sure you have the right version. If the trainer is written for v1.1 and your game has been patched to v1.2 then this is a good reason for the trainer to fail. Trainers are looking for very specific places in memory or the executable and updates tend to shift these locations around. Many times you can reinstall the game back to the level needed to get the trainer to work. Otherwise you will need to wait for a new version of the trainer to be released.