When scrobbling from the Android app works (which isn't always... see my other posting on this), I've found that with very short songs it always fails to scrobble them properly. It sends the initial "Successfully registered now playing", so it temporarily shows up in last.fm as 'Now listening to', but it seems to not send the subsequent "Successfully registered submission" in time. Accordingly, last.fm never records it as a completed listen and doesn't record it in your library.
This is with songs that are about 10s long.