Pieter is exactly right. The current code is extremely conservative with multisig addresses, only counting them as yours if the wallet contains all the private keys.
Loosening that to considering them yours if you have enough keys to spend might happen, although I am worried that might cause vulnerabilities for applications that make the implicit assumption that if they have the key necessary to spend that means nobody else can possibly spend those coins after 6 confirmations. If it is a 1of2 multisig that wouldn't be true.