I'm not sure, but h:title looks like it only selects the title which would not produce much of a TOC.
You could try selecting the default "auto" TOC:
Code:
ebook-convert a.html a.mobi
or
Code:
ebook-convert a.html a.mobi --use-auto-toc
What I typically use is:
Code:
ebook-convert a.html .mobi --output-profile kindle --chapter "//*[name()='h1' or name()='h2' or name()='h3']"
If you get multiple identical TOC entries with this version, replace //*
Code:
ebook-convert a.html .mobi --output-profile kindle --chapter "//h:p[name()='h1' or name()='h2' or name()='h3']"
.