Hi @mself084! As you mentioned your Base Station being nearby and only separated by a floor, perhaps we can eliminate the Range Extender. Just in case the Contact Sensor in question is bouncing between Range Extender and Base Station, let’s unplug the Range Extender and see how both Contact Sensors operate with the Base Station.
Understanding this is only happening to one Contact Sensor, please also check the area for environmental factors. There is not much that could cause this but if there are metal objects or appliances nearby, it may be blocking signal. Additionally, it is always recommended to try ethernet connection with your Base Station, rather than wifi, to improve connection performance. Feel free to let us know how this goes!