If you aren't using the correct bolts you'll have nothing but trouble. Even a tiny bit of play will force them loose, as you've seen. WoundedKnee is right, you must tighten the bolts as you would a drum, not in a consecutive fashion.

Remove all the bolts, put them on using just your fingers. Tighten #1 just a little, then skip over to #3 or #4. Tighten just a little then to #2, then #5; tightening each bolt a little at a time so the chainring is pulled to the spider evenly. If they still come loose, you need new bolts. Or worse, the spider holes are wallowed out. If you ride a lot with loose bolts, that can happen.