Cross referencing #889 which may be related,
The items that won't delete are from feeds from mid-December (the feeds of which have since been deleted). They not only exist in the database, but their `post status` is still marked as `publish` after being "deleted".
It appears that for currently extant/live feeds that the delete button "works" properly for them in that `post status` has been changed to `removed_pf_feed_item` and they don't re-appear in the interface upon refresh. I am surprised that the entire post wasn't deleted from the database however--is this a bug or a feature? I might expect that archiving a post would hide it while still keeping it in the database, but not delete.
Currently using v4.2.1 on WP 4.7.1.