Fistula problems

Hey all. I have asked nurses doctors and techs and never get an answer. My fistula is five years old and has been used five years but my issue is that in the last year I have had to have 15 interventions with it. Narrowing is the problem. I have had two stents put in 6 months ago and the narrowing just moved down my fistula. I ask them why so much and they have no idea. Very frustrating. Does anyone have an answer, is my fistula failing? I have normal to low blood pressure and no other medical problems. I am on no medicine except nausea meds. I have to go back on Thursday. Any impute would be greatly appreciated.