Hi,
I ran into this same issue and I thought I'd resurrect this topic. Apologies if this has been resolved elsewhere. I'm using moodle-1.8.3 (2007021530).
I was seeing the same missing attempt button behavior, but I've found it's related to the debug settings.
For the same student in the same valid quiz conditions (within time/attempts -- set to forever/unlimited):
If I have debugging error messages turned off (Debugs Messages --> NONE, display errors --> unchecked) everything works fine.
If debug error messages are turned on (Debug Messages --> DEBUG, display errors --> checked) the attempt quiz button is not visible.
There are substantial differences in the produced (but not visual ) produced html. In particular the debug version has this extra bit BEFORE the '<!DOCTYPE ...'
<?
xml version="1.0" encoding="UTF-8"?>
<!--
DEBUG: Content-Type: application/
xhtml+xml
-->
leif