I had that problem, too. I kept experimenting with different muzzle placement. I also emailed quite a bit with ThinLine and they were super helpful. They actually ended up sending me a smaller size muzzle and that did the trick! I will admit that it was really frustrating (if I see another bread tie on a muzzle I think my brain might explode) but it ended up being worth it to try to make it work.
I think what the larger issue was that I needed to figure out how to get the hole in the right spot. Once I got that in the right place, she no longer tried to eat out of the side.
That was last summer. When I put it back on her the last time, she was back to eating out of the side. I think I figured out that I had made the crownpiece on her halter bigger, so I made that one hole shorter and the problem was fixed.
I will also say that looking at the muzzle on her while she’s just standing there, it fits much closer to her mouth than the pictures and instructions say that it should, but it works for her and she doesn’t mind it.