Fragment matching not working

Hi all,

I upgraded a TM (around 55,000 TU's) so I could use it as a source for Fragment Matching in Trados 2021. Unfortunately, it's not working. I set the minimum number of words for a TU fragment to 3, but it doesn't find any fragments. They do exist though, because a Concordance Search yields a 6 word match, for example.

Instead I'm getting a few useless Fragment Matches from TM's which have not been upgraded. Is there another setting I need to look at? Thanks!