Warmonger Onyx not unlocking.

[Mikado]

I've beaten the campaign in arcade on Insane 3 separate times already and it still won't unlock. This is really annoying. I wanted to get Seriously 3.0 eventually but I guess that's out of the question..

Is there any way to fix this?

I've beaten the campaign numerous times. Four player co-op, Insane and everything. A lot of people mention that when I searched so that doesn't help. I have a score for every chapter (meanwhile my friend is missing everything from Act 3 Chapter 4 and on but still unlocked it) so that doesn't help either.

[Sanecow]

When you go to start an arcade game, you get to select a chapter. Each chapter has an image indicating what difficulty you have beaten it on. Have you checked that all chapters have the insane image (especially the prologue)?