Humidity, atmospheric condition and the wood used for the handle can all affect the folding mechanism of your Opinel. As wood contracts and expands due to humidity it will hold the blade tighter or looser, this is why the function of your Opinel is not always consistent. If your knife is hard to open due to the above keep it in a cool dry place for a couple of days to give the wood a chance to dry. Some people will place the knife in a ziplock bag with a little uncooked rice overnight. The dry rice will "pull" the moisture out of the wood fairly quickly. See "Smoother opening of blade" to learn how to open a seized or hard to open blade.