look and see if the tensioner moves , see if its in a different position from when you initially tightened it. My timing belt came loose on my race car 3 times during dyno tuning, come to find out my new block has a shitty hole for the timing tensioner bolt that let it tighten down on the tensioner but not enough to keep it in position under a load. To fix i had to add 2 washers to the tensioner bolt to keep the threads from bottoming out.