Because of my native language is not English (=Russian) I create slugs very attentively: collate everything.
To study my hypothesis, just now I specially created keywords in different sequence and added them to the new post and to the old post. And voilà: work right only the tags created first in NextGen! So all my posts created with it’s keywords earlier (before I installed the gallery) don’t relate with images.
If it really is the reason, it’s very bad… :(( Suppose I need to recreate and reinstall to posts all my old keywords… :((( Can it be solved in another way?