Are you running OEM or replacement rings?

I ask because there are two scenarios. Quality cranksets support the rings on the shoulders, and the bolts only do two jobs, act as driving pins and holding them in place on the arms.

However, many replacement rings, and some cranks don't properly support the rings on the crank arms, causing them to depend entirely on the bolts. Because there's always some slop in the bolts (no matter how little) the rings undergo precession, since the load direction rotates with the crank. That causes them to work at the bolts, eventually loosening them.

One thing you might try is some lapping compound between the rings and crank arm faces. That will improve binding between the two, hopefully enough to combat the precession which is the core problem.

FWIW - stronger bolts WON'T help. You have to address the precession or you'll be dealing with this forever. Once you do address it, you'll eliminate movement, and the rings will stay tight forever.