automatic update not working

I have developed an extension for Firefox 3.5, and created an install.rdf 
with updateURL and updateKey.  I have generated a hash from the xpi and 
created an updateHash element with it in the update.rdf.  I have signed the 
update.rdf.  When I upload the xpi and the update.rdf file to my server, and 
I look at the addons window in Firefox and click on "find updates", the 
update is found.  However, the update is not automatically found -- that is, 
when I start Firefox, it is never the case that Firefox tells me that there 
is a new update and asks if it should be installed.  Nor is the new update 
installed automatically.  What should I check to solve this problem? 
Thanks.

Susan