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There are other things to consider, as well. Perhaps the "rubbing" or stimulation of her external genitalia ('private parts') by the texture of a diaper is what she requires to stimulate, relax and start a stream. Try having her wear a smaller size of underwear to reproduce the sensation. If this issue is not dealt with, you could run the risk of over-extending the bladder beyond its normal capacity. She may need to be taught other exercises similar to those you learned in prenatal classes... start a stream, stop and hold, release and repeat. Again, this would help her identify the muscles needed for passing water and how to help relax them.