That is odd to hear @SLCloster! As long as the Sensor and cover used is the same generation/ model, it should fit. As it sounds like this is so, instead of sliding the cover straight on, try sliding the latching part of the cover on first, then push the rest of the cover closed, to see if this results in a secure closing. I recommend also ensuring nothing inside the Sensor is in the way of the cover area, and that the cover is right side up. Feel free to let us know how this goes! 