Normally, I use only the ios app for studying. I finished a list on July 29th (http://www.skritter.cn/vocab/listsect?list=197471450&sect=0), however, about 1/3 of the list never showed up for studying; only today, when I was studying on the web app, these items came suddenly up(e.g. Bässe, Verstärker, 3D Klang,...). In the meantime, I had added almost 100 items from another list and got my items due down to zero a couple of times.
One other strange behavior is on my statistics: There is a gap of around 75 items between Character writings added and Character Definition etc. added. This gap keeps the same many weeks already. Does this mean that I have 75 characters somewhere, where I am studying the writing, but never study the other 3 categories? I always have all 4 categories enabled, I never switch any of them off.

same issue here...

There are a series of fixes coming out with the next app that should fix some sync issues that have been coming up.

For statistics, this is due to how character items are counted. If you study a word, but not its characters, Skritter considers those character writings learned, because you're studying them each individually. Same with tones and readings. Not so for definitions. If you study a word definition, you're not also studying each character definition. So they are not counted as learned. Did you always have 'also add characters' on?

Scott, I always have 'also add character' on, never switched it off. I never had this in the past, only in the recent weeks/months. Therefore, I mentioned it here, as it might be the result from some sync problem.
Items not added: When I first added the items from the list, I brought down the 'items due' to zero on the same day, although there were quite a few items in the list, which never showed up on this very day (not even one of the 4 categories). I also realized, that Skritter didn't pick the items in a row, but these "no-show" items were randomly scattered over the list; these are now the items, which only showed up more than a week later on the web app (but I am not sure, whether this is a co-incidence or has really something to do with web vs. ios app).