What Causes Recurring Urinary Tract Infections?

My 6-yr old daughter has had a history of recurring urine infections related to a voiding disfunction. Although she presents discomfort when peeing with a burning sensation, most of the times the direct tests (at the doctor's office here in Germany) do not show a urine infection. Only when the urine is actually sent to the laboratory does the test show a number of bacteria in the urine (e-coli, other intestinal bacteria). Even after an antibiotics treatment, the discomfort and sensations of the urge to pee return after a couple of weeks. We are at wits end! Thanks