It sounds like the animation itself has a problem.
I've had the issue before where the animation isn't correct or hasn't exported correctly, as soon as test game is run the model moved very quickly in to the ground with animation not working. If it's in a loop it will disappear as soon as you run the game.
PC - Windows 7 64bit, Intel i5, 12Gb Ram, 1TB WD Black, Evga Gefore 760gtx 3GB
Laptop - Windows 7 32Bit, Intel Core 2 Quad Q9000, 4GB Ram, 500Gb WD, ATI Mobility Radeon HD 4850