I think there's a significant problem with the iOS app and single character words (especially as new words). When I open up the iOS app, it says I have something like 500 reviews or so, and then when I complete one or two words, it'll drop by about three to four hundred. I was curious about this, so I started reviewing in the web app to find out what was going on. Sure enough, it looks like I had about... well, I'm currently adding my 106th new word, of which a disproportionate number have been single-charactered. I've seen some from a good 5 lists back, that I've had completed for about two weeks now. I don't recall ever seeing a single character word appear as a new word in the entire time I've been using the mobile app, but my memory isn't really that good.
I'm not sure if this problem extends to single character words on review, but I'll notify you if I see them as reviews just fine.