Crank shaft plly

First of all determine if the "harmonic balancer" (pulley) is worn, especially in the keyway. Is the key itself worn or intact? Then you want to look at the threads on the bolt and the shaft to make sure they are not stripped. Do you have the correct bolt and the big washer?

If everything is ok, then reassemble and use "Loctite"

If the key or keyway is bad replace.

If the threads are screwed up you might do as I have done and retap the hole and use a slightly bigger standard bolt. Loctite is important. If you notice on the original bolt it is on there as well. (Or some honda equivalent)