Hyundai repair

I have a 1994 Hyundai Scoupe with a 1.5 SOHC (3 valve per cylinder) 4 cylinder with 110,000 miles. The engine had a bad head (several bent valves, small gouge in cam, broken pin for cam gear), but the bottom end was good and the timing belt wasn't broken. I removed the head and had it completely rebuilt and installed a new cam gear, timing belt, and tensioner. The engine is reassembled now and I'm having a problem with the belt. I turn the motor over by hand and the timing belt slowly walks off of the gear. Both gears are straight and lined up. The machinest has no idea what the problem could be and neither does anyone else that I ask. I've used the same machine shop for years and never had a problem, but I'm almost certain that the problem must be in the head. Any help would be greatly appreciated, thanks.