One level 3 heading number is wrong - but all others are fine

[Dr Wu]

I'm using Word's built-in heading styles. I attached them to my multi-level list numbering. One (and only one) of the level 3 heading numbers in the document is wrong. It is wrong in the body of the document, although it is correct in the automatic table of contents. Here's a summary of the heading numbers in this part of the document:

5
5.1
5.1.1
5.1.2


5.2
5.2.2

It's this last number, 5.2.2 which is wrong. It should be 5.2.1 and it does appear as 5.2.1 in the TOC, but not in the body of the document. Do I simply need to refresh the numbering? If so how do I do that? By the way, the level 3 heading numbers further down in the documnet (eg. 6.1.1, etc.) are all correct.

Thanks