When the track is warped, cracked, broken or bumpy. In these cases, the track may need replacement, and that is best left to asliding door professional. Furthermore, you can try one of our aluminum “SLIDING DOOR TRACK COVERS”. They glue right to the door threshold and cover the existing rail while adding height so the screen door has nice, straight rail to ride on. It comes in 3, 4, 6 and 8 foot lengths in white, silver and bronze.