Suggest Treatment For Sliding Knee Caps Which Is Causing Pain

hi, I am 17 years old male. I am suffering from my knees. I feel pain when I bend my knees and I can't bend my knee completely. I feel my knee isn't normal. I am an athlete by the way I play soccer and I exercise at the gym. I visited many doctors. mostly, all of them said I am ok. But one doctor told me that I am suffering from sliding knee caps. What should I do ?? Thanks