Sounds like a known issue first reported back when FileMaker 10 was the latest version. This specific bug may have been fixed with the release of FileMaker 11 and thus not apply to your situation, but if so, I've received no reports that it was fixed and the bug specifically references Snow Leopard.
There is some information on a fix that may work for you if this is the same issue.
For More Information see: Filemaker 9
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html