If you try to use autocompletition within a non zul tag nothing is shown.

Try for example

<h:div>
<combo[ctrl+space]
</h:div>

"No default proposals" is shown.
If you add a zk div inside:

<h:div>
<div>
<combo[ctrl+space]
</div>
</h:div>

the correct autocompletition is shown.
But also in the <h: tags (or n: tags) the autocompletition should work