Portal Script / Relationships

I'm very new to Filemaker - i've worked my way through most of what i need and tried to hire a developer to finish the rest but the brief wasn't clear and it wasn't delivered so i thought i'd try to finish it on my own.

I basically have a campaign database. It does a LOT of different things, most of which is working. The one thing I can't get to work is assignings contacts to campaigns - as contacts may be assigned to multiple campaigns and vice versa.

I have a layout for campaigns with a portal showing contacts, and a script trigger to let me select contacts from another layout w/ tick boxes. This is working, but only a one-to-one (i.e. i can assign contact 1 to a campaign, but if i assign that contact to another campaign the original assignment disappears).

I have a join table for this purpose, and if manually input IDs for contacts and campaigns into a form layout for that table, it works and contacts are correctl assigned to multiple campaigns.

This leads me to believe that the issue is the script that assigns the contacts. looking into that script (again, this is triggered from the campaign layout, and this is the bit that was started by a developer but not finished), it simply assigns a campaign id to each contact that was selected.

i can conceptually get what needs to be done but i'm not sure if the issue is in the portal, relationships or scripts.

it's the only thing that's not working on the database (and it's taken me ages to get it just right), but i'd also like to learn what's wrong with it so i can at least learn something!

i've attached an example of the file with most of the data removed for reference and to alleviate my frustrations if anyone would be interested in having a look.

test_db.fp7.zip