What size bolts are they and how much thread length is there on the bolt? A lot of BB designs don't have enough thread engagement to adequately secure the bolt no matter how tight you do them up. Without boring you with bolt theory, any bolt, regardless of length, will only engage a maximum of 3 threads. The bolt needs to elastically stretch in order for it to hold, therefore the longer the better. There is also friction between the head of the bolt and the mating surface which needs to be overcome before it can undo. It's a ratio thing so if you have a short stumpy bolt with a large dia then it's never going to stretch enough to hold. Over-tightening it will deform one, the other, or both parts and still undo. Some sort of thread lock will work for a while (I recommend stud lock). Ensure it cures for the recommended time before using. What I have done before is tighten as normal but leave the 8mm allen key in the head of the bolt and align it with the crank. Tape it around the crank in a couple of spots and it can't come undone. This only works with cranks shaped so they don't catch your feet.