That does not sound good.

Can you use a longer bolt to engage more threads?

Preload is everything. If the bolt does not holed the pulley on with enough tension it will try and move on the crank. This beats up the key and will loosen the bolt.

When a fastener has enough preload the friction forces on the pulley will always be higher than the engines vibration forces.

Try and use a longer shank bolt. I know that might mean going to an early design. I don't think these problems are so much a bolt problem as they are a preload problem. As long as you can get the bolt to the new torque level you will be fine.

Also loctite will change the friction of the bolt and affect the preload level. Do the torque instruction specify lightly oiled threads on the bolt?

Kevin