Connect existing Modernbill/Plesk Billing accounts to existing Plesk 9.0.1

[Spazholio]

Having migrated from Modernbill 5.6 to 6.0 (Plesk Billing) and from Plesk 8.6 to 9.0.1, I can't seem to find a way to link up the two accounts via SSO. I set up a new account, and they show up in both MB and Plesk, and their balance from MB shows in the Plesk screen - SSO works as intended. But for the customers that existed BEFORE the migration, it doesn't. I don't know what I'm doing wrong, and can find very little information online. Could someone tell me what I'm doing wrong? Thanks!