Yes, the same thing happened to me. Not 6 stories, but 4 were totally ignored, but my last 2 got curated again. I also suspect that Medium is playing around with curation. I don’t think they raised the bar or anything. Lot of people say that it’s basically autocuration now. If you check out your stories’ html, sometimes you can see the topics suggested to your piece to be curated in. I suppose after that someone needs to ‘okay’ or reject it for curation.
Curation doesn’t bring too many extra views, but without it you barely have a chance to reach a wider audience – you lose the potential.