OK, I'm trying a new approach, given that my retention rate has dropped a bit and I seem to have quite a large backlog of old items to review (around 500-700 a day). As a result, I've paused all my lists, so no new items are being added, and I'm going to spend the next few weeks just going through what remains. When I have more thoroughly mastered the backlog of tricky items, I'll start adding some new items again.
I suspect that when my backlog gets too large, there's simply too much old stuff to remember to have new stuff added, and so the time spent learning is not too effective.
Anyone else tried this approach? There's been some discussion on the forums about making the SRS algorithms more intelligent; but I wonder if what I personally need is to make my own approach to using Skritter more intelligent... And this seems like a step in the right direction.