Sometimes a bladder irritant like a food allergy can affect this, or sugar, corn products, etc. Not sure if playing with the diet would help or not.
Fresh ground flaxseed can provide a plant based estrogen, which sometimes can help. Hylands has some homeopathic remedies called incontinence or Bedwetting, which might be available where you are, and might help.
There might be an underlying bacterial issue that the test aren't picking up? Some cranberry or d-mannose may or maay not be helful?
Just throwing out things to try and eliminate.
My girl had an extra band of tissue running along her vagina which caused urine to collect and caused some bladder problems - all sorts of weird problems are out there.