Why is my instax camera underexposing photos on house setting?

Hey so I was just wondering whether anyone has had this problem before. My current instax film started printing perfectly (the two bright photos with people in them) and have just gotten darker as the film goes. When the previous film was in, it jammed for two photos and then printed a few intermittently black or white with some absolutely fine. I thought the issue was the film as when I changed the film as the first two were fine. All of them were taken with the house setting so I’m really stumped with where I can go with this. Has anyone else had this problem because it’s such a waste of film? Thanks in advance!