You need unique header names in each header level or the toc link leads to the first occurence of this header name.

If you have a header structure like this:

1. text1
a. subtext1
b. subtext2
2. text2
a. subtext1
b. subtext2
...

a click on 2.a or 2.b (etc.) leads to the first appearance of this header text (2a->1a;2b->1b).
So in this example you are not able to get the link 2.a. working in the toc if you do not change its header name to a unique one.