Why is my film disappearing?

I’ve got the instax mini 9, which I bought brand new. I appear to be having s problem where my film is just disappearing!!! I will put a full cartridge in and over time the counter (which tells you how’s much film you have) counts down even though I haven’t taken the photos. I took the cartridge out to see I’m the problem is the counter but there is no film. Someone help !!!