Whenever you enter data and then can't find it, a possible explanation is that your file is damaged. Try recovering the file--which rebuilds all the indexes--among other things and see if the recovered copy works correctly. If it does, replace your file with an undamaged back up. Don't rely on the recovered copy to work correctly as the "repair" may have altered the design of your database in unexpected ways.
If you have a functional copy of filemaker that is newer, it's worth a try to see what you get when you attempt converting the data base to the newer version. Launch FMP 8 and use File | Open to find and open your FMP 6 file. It will create a new converted copy of your file while leaving your original copy unchanged. Test your new copy and see if it works correctly. If it does not, you may find that only a few simple "fixes" are needed to complete the conversion.