Futon mattresses are notorious for slipping off the frame. Sometimes this problem is caused by an inadequate frame, or an old or bent frame. Sometimes the culprit is the material from which the mattress is made. Regardless, it can be extremely frustrating and uncomfortable to use the futon as a sofa when the mattress keeps coming off. There are a few easy fixes you can try before resorting to purchasing a new futon frame.
Purchase some nonslip material from your local home store. It comes in a roll and is usually placed between hardwood floors and area rugs to keep the rug from slipping when walked on. Place the material between the futon frame and mattress. It can be cut to size, and will remain unnoticeable from the front of the futon.
Purchase a cover for your mattress. Most futon mattresses come in a basic cotton or polyester blend fabric, which slips easily against the metal or wood on a futon frame. Covers will not only protect the mattress from stains, but they are made in a variety of comfortable materials. Choose an appropriate fabric for your frame style.
Tighten the bolts on your futon frame at least every six months. Part of your slipping issue could be a result of a sagging frame. You might not even realize your frame is no longer resting in the appropriate position. Try adjusting it, then maintain the correct angle.