I've posted about this in the iOS WordPress forums, but I thought I'd ask here incase the answer is obvious.
I've installed Simply Exclude on my site, for the purpose of removing posts of certain categories from my homepage (they live only in their own category).
The problem is, this seems to also exclude them from being seen via the WordPress iOS app. I'm not sure how the app gets it's list of posts, but apparently in excluding certain posts from the main blog index, it also excludes them from the iOS app, even if they're in a draft state (so obviously, they wouldn't show up on the blog anyway since they're unpublished).
Changing the category to one that's not excluded from the homepage allows them to be seen by the iOS app. Changing them back to an excluded category causes them to be hidden again.