Hi, My Name Is Susan. My 12 Year Old Dog

Hi, my name is Susan. My 12 year old dog has recently been diagnosed with CHF and placed on Lasix. She weighs 19 lbs. and Vet prescribed 12.5 mg cut in half and each half administered twice per day. It has not increased her urinary output, and I am wondering if the dose is too low, or maybe she has no excess fluid to excrete?