If a "simple" pelvic x ray with contrast fails to reveal a suspected angulation, torsion, intussusception, or other mechanical problem, I suspect you may be treated to the multiple pleasures of a "dynamic defocography." As the name implies, after prep, you get your pouch filled with a barium paste with an appliance like one caulks their bathtub with, then sits on a commode trying to empty their pouch while being observed via fluoroscope. To my knowledge, this often helps nail down with a definitive diagnosis.