What TB said. Check the bottle first and check closely. Don't remember who it was, or where in the 9,000 pages in this thread it was, but there was a whole conversation about a leak and it turning out to be a small crack in one bottle, right where the neck flares out to become the top of the bottle itself. There was also a bit of conversation about how to lengthen the life of the bottle some by adding some spacers in location around the bottle to stop bottle movement and shifting when we press on it to feed.
If its not, again, like TB said, I remember posting when I opened up my original pkg and asking on the forum here what the heck that little thing was. If ya don't know, the unused one could very easily escape and never be seen again. The one in my device has made a break for it once. I had done a complete cleaning. Afterwards was inserting a bottle, and instead of a smooth tightening of the bottle, it seemed to be getting hung up on something. Found that itty bitty thing right at the edge of where you insert the bottle. I broke a tooth pick in half, flatten the end of the pick on the counter and used it to work the seal back down into place. First one side then the other, repeat.
Hope that's all it is. Good luck.
Mac