Hi, I am working in a document where my headings are numbered

H1 = 1
H2 = 1.1
H3 = 1.1.1
H4 = 1.1.1.1

On my heading 3 when I got to the point where it should number 5.1.2, it goes to 6.0.1. I've tried overriding but I get an error message saying that the number I'm changing is less than the previous number.

I have also gone through every line in between to make sure there is nothing randomly selected as a header and looked up every possible way to fix this. Anyone ever have this problem and a possible solution? Thanks in advance.