12 yr old constant mucus in her airways

hi my 12 yr old has just spent quite a few weeks in hospital undergoing tests.
she first went in for laryingitis which she had had for weeks,when they operated to have a look they found her windpipe was really inflamed and full of black mucus and further down her airways to her lungs were the same.
Since then and after having introvenis antibiotics she has been back in 3 ties with the same thing,the doctors seem to think there is something underlying for it to keep reaccuring but ost tests are coing back clear.
pleases could anyone help!