A picture would help, but I'll have a quick go at it.
1) Are the pants simcloth or modeled and attached to a bone? If they are simcloth, then look into collision tolerances.
2) If the pants are modeled and attached to a bone, they may be attached to the wrong bone. The otom of the pant leg may need to be attached to the foot rather than the leg or weighted between the two.
Scott