Well, let's see . . . step 1 is to take comparable caliper measurements between your magazine and literally any other compatible brand magazine you can find. Maybe try phoning a friend.
Front and rear wall width measurements, and an overall length measurement if your calipers are long enough.
Step 2 is to test fit this additional magazine.
Step 3 involves some deduction;
3A - if 2nd magazine fits correctly, AND measures smaller, then it's probably a magazine issue
3B - if 2nd magazine also fits poorly, AND measurements equally, then it's probably a bottom metal issue