If you talk a look at the chair from the underside on the side where the chair is not moving the is a plastic assembly and the cap to that assembly often comes lose and is what causes the seat not to move forward because the cap keeps the mechanism in place when the cap comes off the mechanism moves out of place and the slider does not move.
Just had this same problem happen to me. Good thing I'm not the only one haha. The little white cap thing is very hard to get back in, but it is possible without removing the seat. A 12mm wrench will fit the nut on the cap. I used an offset wrench to tighten it back down after startin it with my fingers. Hope this helps anyone with this problem!