I assume they're square taper interface... Your crank arm might be bottomed out. If the spindle is going too far in, because the hole in the crank arm is worn out, then the bolt can't actually fully tighten against the crank. This will make your crank arm wiggle, and will back the bolt out as you ride.
You might not be able to see that your crank is messed up, because it might not be rounded out at all, just stretched from over-tightening.