accuracy of MRI's

Can anyone advise about whether MRI's always pick up problems.
My Daughter has bladder and bowel problems, after 5 years we have got nowhere. It was suspected she had spina bifida occulta at one time but an MRI of her spine has ruled that out. She hasn't had an MRI of her head. My daughter was toilet trained for 3 years, but when she was 6 she developed recurrent UTI's and urge incontinence with frequency problems, a year or so later she developed chronic constipation maybe as a result of oxybutynin used for bladder.
I am not convinced that one MRI or spine can rule out all neurological problems - can anyone advise?