If a bug tracker has a lot of watches (for example: http://launchpad.net/bugs/bugtrackers/debbugs or http://launchpad.net/bugs/bugtrackers/gnome-bugs) it can very quickly get out of date, with some watches not being updated for well over the standard 24 hour period that we'd normally expect all watches to be checked over.
This may be because checkwatches picks more-or-less at random from the list of watches that have already been updated but which still need updating (watches which have just been created should *always* be updated within a few checkwatches cycles, depending on volume, though we should do some research to discover whether this is actually the case).
To fix this, we should make checkwatches select the 'old' bug watches to update in a given run in order of date last checked, oldest first. Furthermore it may be worth looking into bug 302529 and bug 67480, though these are non-trivial to fix given the complexity of checkwatches at this point in time.
This was fixed in r8623.