Holy moley! Those are some giant needles! 12mm is equivalent to US17, right?
Anyway, here's my 2 cents:
I think it's the combination of a huge needle and stitch manipulation (cable). I recommend that you snug up your stitches when working the cables. Try to work the cables with just the tips of the needles. Go slow. Make sure not to stretch out the yarn.
What yarn are you using? Some yarns are just more slippery and difficult to manage...but it can be done!
Good luck! Check back with us! Let us know how you manage!