I had the same problem with my M400 watch. The top right button was jammed. My solution was this: open the watch and remove the pcb and everything from the inside. Take some tweezers and dip them in a fine mechanics oil (Ballistol for example, or any thin oil that doesn't harden. But please no WD40!). Then push the button and apply the oil on the pin that pushes the contact on the pcb. Try to press the button some times until it goes smoothly. Than reassamble the watch. Be careful: collect any drips of oil before the assembly.