A couple of thoughts, vibration may have loosened them to the point of release, blue loctite will take care of that, just a drop on each. Do Not Use red loctite unless you want them permenent. If the screws are stripped then replace, If the holes are stripped then a retap or helicoil is in order, retap go with a slightly larger diameter and longer screw, helicoil will need a smaller screw I think. These are a couple of things I have done with repairing old motorcycles, but should translate to any vibration prone machinery.