Some keywords ranked well still, but some are not,why?

[flatware]

I have 3 sites, before algo changed, their keywords all ranked well, then sometimes well, sometimes bad,

till now, there are a long time, Some keywords ranked well still, but some are not,even seems it is filtered, I tested it by google-watch.

I try to reduce keyword density, but the page content indexed is updated, rank is no change, anybody could give some clues?

by the way, I watch the other site all the time, their keyword density is very large, but their keyword rank is very well all the time, and isn't filtered at all.