I haven't experienced this issue before but the first thing that comes to my mind is is the battery low/has the vehicle gone into deep sleep mode? My 2018's battery was changed a few days ago after a host of issues occurred. During that time I regularly got the message from the Lincoln Way app that the vehicle has gone into deep sleep mode and remote features were unavailable (starting, locking/unlocking from the app). The infotainment screen also displayed deep sleep mode was active. I don't know if some of those features would be unavailable if you try to remote start from the key itself rather than through the app. I don't know if a 2015 is compatible with the app but one thing I noticed is when the vehicle enters deep sleep mode, (and assuming you configured your settings this way), if you try to lock or unlock by using the handles only (not using the key fob at all), only the driver's will work (i.e. I cannot lock or unlock the car from the rear and passenger door but only the driver's). After I ran mine for a few minutes and tried again deep sleep mode turned off and I could use every other door to lock and unlock. Maybe this is a way to quickly tell if deep sleep mode is on, and this mode might deactivate other features such as remote starting from the key.I have been using my remote start without failure since I bought the car (2015 Lincoln MKC 2.3 Ecoboost) almost 2 years ago. This past Tuesday it just stopped working. The key fob will open and close the car like it normaly does except to start the car remotely. What might be the issue?