On the inside of the frame are two square holes, with small plastic parts sticking through. These are the nib springs which are meant to retract when the fold mechanism is activated, allowing the frame to slide down on itself.
Sometimes one of these buttons may stick, blocking the fold. Please try pushing it in with a fingers or a pen, to free it up. If it moves freely but the frame still does not release, please pick up the buggy by the handle and give it a firm shake. It may be that the folding cable inside the handle has caught, and a shake could release this.
If none of these work, please drop us a line via the contact us link.