Pull the sensor out of the bottle. It will likely have corrosion/buildup on the sensor blades. Get a nail file, and sand off all that white/yellow powdery stuff. Then let it sit in baking soda water for a while. Make sure it is clean, and put it back in the bottle. The corrosion/buildup blocks the electronic current from touching the water, so the sensor thinks there is no water in the bottle. Since you have water, this is the problem.
I would post pics, but I deleted them long ago.