There are a couple of situations where Launchpad hides some bug reports from searches that really shouldn’t be hidden in my opinion.
I recently fixed one of these cases - where if you were to search a distribution series, Maverick for example, for bug tasks via the API you would receive an empty collection. Of course, this makes no sense as Maverick is being actively worked on and it has bug tasks targeted to it. The bug tasks are hidden because omit_targeted parameter had a default value of True. While it is easy to work around by setting omit_targeted to False, this isn’t easily discoverable nor should it even be necessary. If you are searching tasks for a series you want to see the targeted ones!
Anyway, its fixed now but you need to be using the devel version of the API.