Three replacement parts are listed as suiting Alulite® patio doors and windows: a 25kg front adjustable patio door carriage and roller, the matching 25kg rear adjustable version, and a keyed window catch and spring. A front and rear pair like that is a sign the carriages are what give out. Each sliding panel runs on two, and the adjuster position at one end or the other is what sets the door height and squares it in the frame. Before ordering, take one carriage out and look at which end the adjustment screw sits, because a front adjustable unit will not fit a rear adjustable pocket. Check the roller diameter as well, and weigh up the door, since both units are rated to 25kg and a heavier glazed panel needs more than that.