Spray some contact cleaner into the kill button/stop button area. If that doesn't work then you may have to take the handle apart to get to the metal tab that the switch lever ramps up on to push the tab to make contact with the ground. I think the small "hook" at the bottom end of #22 in the illustration is the ground or stop contact for the MS250 which is similar to a lot of Stihl products. There's a ground wire at the handle attached near the #22 tab and that wire runs to the engine case to ground. When you try to stop the saw you might try wiggling the stop button to see if it is just slightly out of alignment with the ground contact if so then you will have to go in and reposition the tab or just wiggle the stop button all of the time

Mike