I've got some of that too and happened with bins where I mostly used corr. cardboard. I had a batch of cardboard that for some reason took forever to breakdown; even when wet, not too many mites went there. I think the ccb had been sprayed with something to deter bugs and vermin. These bins are also slow with the population increase (could also be just my impression).
Adding shredded newspaper usually helped.
Bottom line though: if you still have undigested bedding, I'd say it hasn't finished composting.