I've been fiddling with FileMaker since version 5 but one thing that has always bugged me is why lookup functions based on relationships work for months, then stop working. Most of the time I haven't altered anything. I spend hours fiddling around and end up having to half rebuild the file. Sometimes if I reconnect a lookup it doesn't always refresh the data on every change. I can test four matches to Name > ID, three will refresh and one won't, then when I repeat half won't refresh. Is there a particular way to troubleshoot lookups based on relationships to ensure everything is nulled or refreshed?

According to your details you're on FM7 - well known to be buggy.

Could be indexes on the target field that are corrupt?

I've just upgraded to FMP 11, but the db was built under 7. I'll try the indexing option in the future. Does turning it off and on clear the previous index or is there a more involved method?

Save a compressed copy, too. That clears up a lot of problems.