This should work, I use it all the time.
Check your relationship and the values in each key. Check the table occurrence references for the portal and the layout it's placed on. I suspect something in one of those areas is causing your problem.
When you examine your relationship graph, you should see something that looks like this:
MainTable::GlobalField = MainTable2::NonGlobalField
Your layout should refer to MainTable and your portal should refer to MainTable2. I'd also make sure that the two key fields are both the same type.
Thanks, that worked!