A lot of game jams won't allow you to submit games you worked on previously, but you might be able to find some who don't care as much. The smaller the game jam, the more likely people will leave feedback on your game. Game Off has so many people involved, it can be overwhelming to play/vote on everything.
My only other suggestion would be to share the game elsewhere, like Facebook, Twitter, and Reddit. Especially on Reddit can you find some people willing to check out your prototype and share feedback. Hope that helps!