Found a weird WordPress bug today. I exported ALL from my old WP hosted blog and imported everything here. It all went fine except all the Post counts for imported Tags in the Tags screen were zero. I found this old post with a similar problem but no solution.
I figured it just needed a nudge to force it to recount the Tags, so simply did a Bulk Edit and added a dummy tag to all my Posts. Checked the Tags and bingo! Then I just deleted the dummy tag.