I've had both sides not open on different occasions. Both were caused by over-stuffing the space. If you overfilled the bag and you don't hear the lock mechanism click there is something pressing up against the plastic cover and bowing the rod from the two hooks holding the bag closed. In both cases I had to get the weight shifted inside the bag so that the mechanism would move properly. In one instance, I put my bike on it's side and lifted it back up (a couple of times) to get the jacket and case I stuffed in that side to move enough internally that it finally opened. The second time, I did the same thing but had to lay the bike on both sides several times before it finally let go...that one was a cooler and a blanket...I thought it would never budge.