Timing belt problem

I am asking for help for my dad, who is a mechanic. He has a 99 escort zx2 2.0 dohc in his shop. He said it keeps chewing up timing belts and he can't figure out why. He said he put the belt on, after installed and driven
the belt moves forward and get chewed up. Does any one know what is wrong? PLEASE HELP. Thank You. I hope I explained that good enough.