How did you cut up the skirt? do you have a middle hip part going all the way down and two thighs?, if so you are likely to have splitting occur when you are bending thighs. I usually make only an abdomen and hip group and take care of the posing with 7 morphs: left thigh sideways, right thigh sideways, left and right bend forward, left and right bend backwards and a sitting morphs. That usually covers most posing situations.

Dominique Digital Cats Media