A futon is a space saver because it can serve as both a bed and a couch. Not only do you save on an extra piece of furniture, but folding the frame into a couch affords you more space in the room. However, frames are sometimes tough to fold. If your futon won't fold, there are some troubleshooting measures you can take to try to diagnose and correct the problem.
Inspect the mattress. Thinner cotton mattresses can remain in one piece and still fold, but foam and innerspring mattresses are too thick to fold unless segmented. Thicker bi-fold mattresses are normally constructed with two to three partially attached segments that allow the mattresses to bend in the middle. It is possible that the back of the mattress is facing up, so that you are essentially trying to bend the mattress the wrong way.
Lift the mattress and inspect the frame. Some basic bi-fold frames have a pin in the center of the frame that keeps the frame flat; if you don't remove the pin first, you won't be able to fold up the frame. Also, see how heavy or light the mattress is; if you can't lift it up, then the weight could be preventing you from folding the frame.
Check the clevis and lock pins. Also known as hinge pins, these mechanisms connect the two main panels of the frame while allowing them to fold and unfold. The clevis pin sits in a hole in both of the inside corners of the frame; a lock pin is inserted through a hole in the clevis pin itself. Check that both pins and the accompanying washer are not loose, missing or jamming up the frame.
Check all bolts and tighten those that are loose. If any along the folding mechanisms on both sides seem too tight, to the point of not letting parts move, loosen them slightly, just enough to get the part moving. You do not want to loosen it too much, because you risk causing the frame to fall apart or having the loosened part jam the folding mechanism.
Check the rollers along both sides; ensure there is no debris blocking them and that the pins connecting them to the frame are not loose.
Try lifting the frame again. It is possible that you are trying to push the frame in too soon; in order to properly re-engage the rollers and re-fold the frame, lift up from the center of the frame's edge, and push only when you feel it start to move forward.
Push the frame all the way forward when you have it partially folded while pulling either the top edge of the mattress or the side of the back panel, depending on the model. Getting the frame into a somewhat folded position isn't good enough, because it may just slide forward again. Push and pull the frame and mattress until the panels seem like they have clicked into place.