Simple and obvious question: But are you sure they are in fact related?
Here's quick check for understanding:
It sounds to me like you are entering find mode, entering data in a portal's field and then not getting the records you expected when you perform the find.
Is that the problem?
If so, keep in mind that when you perform a find to search for records in a related table such as those displayed in a portal, you are telling filemaker. "find all the records in the layout's table that have at least one related record that match this criteria." It then finds all such records, but when you look at the portals, you'll see all the related portal records--not just those portal records that match your criteria. This is how filemaker is designed to work. If you don't want to see the portal records that don't match the criteria, you'll need to set up a portal filter (Easier in filemaker 11, but can be done in earlier versions) or perform the find on a layout based on your portal records instead of whatever table you current layout is based on.