MS Word Styles & TOC numbering

I have inherited a 34 page word document that appears to have had manual headings and numbering added, alongside a manual TOC. I want to ensure the numbering works automatically and in turn the TOC can be updated automatically; to avoid any paragraph numbering inaccuracies. I am familiar with styles and TOCs but for some reason, I cannot get the sub-headings numbering to follow on from the Heading 1 numbering, only linking to the preceding sub heading e.g. 1. Heading One, 1.1 Sub Heading 1.2 Sub Heading, 2. Heading Two - next should be sub-heading 2.1, 2.2 etc but it is going 1.3, 1.4 etc