Jessica,

     It might be as simple as:  mom is left-handed, and right-hand slips
during pumping.  However, if you could provide more information about the
type of pump, attachment, etc, there could be other explanations.  Also,
soreness on right nipple could be related to plugged ducts.  Where exactly is
the pain?

     Another simple solution could be the pressure gauge.  Has she lowered
the level of pressure on the pump?   Debbie Albert, IBCLC