ME2 Justicar achievement not unlocking

I'll just go and add another achievement complaint into the mix...

The Justicar achievement in ME2 is not unlocking for me no matter what I do. I have tried several times now but no joy. It's my laat achievement so I would very much like to obtain it. The other squad-based achievements unlocked without any issues.

I am playing on Xbox One.

Anyone else had this problem?