Are you doing a back wrap cross carry or a reinforced rucksack carry? It sounds from your description like you might be doing the second one.
In the back wrap cross carry the first pass across the baby comes *under* your arms, the ruck straps go over your shoulders and back down across the baby, spreading over the back/bum and bunching under the legs.
In the reinforced ruck the first pass comes *over* your shoulders, then the ruck straps come under your arms and cross over the baby, spreading over the bum and bunching under the legs.
If you are doing a reinforced ruck I think switching to a BWCC will fix your problems. By bringing the passes over your shoulders you will be able to trap the baby's arms under them and secure her into the wrap more. Also I don't know how big your baby is but I notice if the baby is too small there is not room to spread an underarm cross pass before it goes under the leg, and it always ends up bunched. By having the cross pass come down over your shoulder first there will be more room for it to spread before it has to go under the baby's leg. HTH
Me, DH, DD1 (5/2009) and DD2 (10/2011).
I'm not crunchy. I'm evidence-based.
Vaccines save lives.