The spring has an arm that fits into a cut out in the cover. The arm has probably fallen out of its spot. I've seen this happen a few times before. I have pushed the pin that holds the two parts of the cap together out and reinstalled the spring propperly with good results before. You'll see how it is supposed to go. Look at the cap that still works as a guide if you need to. If that doesnt work, call Aimpoint. I've had them replace a cover that wouldnt hold the arm in place.