Not sure what I can add here over and above the advice you're probably already getting internally.
If it's any consolation - this is a common issue facing new hires to big4 consulting. A few months on the bench is quite normal and most people who experience this go on to have a successful run.
However, after 6 months you're right to be a little concerned. It's almost certainly not a matter of skills/experience though. It's probably down to bad luck...but perhaps also consider how you're presenting yourself.
After 6 months I'd suggest it's OK to start asking staffing managers/partners directly what they think you could be doing differently. I'd be interested to hear their thoughts.
Final point - make sure you take advantage of any and all training going whilst you can.