[Sparkle] How to debug an appcast?

[Ben Willmore]

I just added Sparkle support to my application, and am struggling to
determine whether it's actually working or not. Here's the evidence:

* My appcast appears to be correct, and more or less validates (except
namespace problems which are expected). It's:
<http://willmore.eu/software/isolator/allversions.xml>

* From my local machine, everything is working fine. I.E. If I delete
my preferences, run an old version of the program
<http://willmore.eu/software/Isolator-1.5beta3.dmg>, switch the
preferences so it subscribes to the 'all versions' feed above, and
then either check for updates, or just wait for the auto-check, I get
a Sparkle window and the new version downloads correctly. And, to
match, I get correct-looking lines in my server log:

xx.xx.xx.xx - - [19/Mar/2008:03:36:42 -0700] "GET
/software/isolator/allversions.xml HTTP/1.1" 200 2458 "-"
"Isolator/1.5beta3 Sparkle/1.5b1"
xx.xx.xx.xx - - [19/Mar/2008:03:36:46 -0700] "GET
/software/download/Isolator-1.5beta4.dmg HTTP/1.1" 200 1724700 "-"
"CFNetwork/221.5"

* So, it all looks good so far... but, when I look through my server
logs, I can see (a) that the GET for allversions.xml is almost never
followed by a GET for the updated version. Also, the server logs show
that many users are still using older versions.

I can't think how to debug this problem. It works seamlessly for me,
but users never get updated as far as I can tell. One possibility is
that everyone hits 'Skip this version' or 'Remind me later', but it
seems surprising that 100 people would *all* do that.

Any advice on how I can debug this?

Many thanks,

Ben