I had the same problem. I discovered that at step 87 in the manual, I had flipped the yellow T-shaped piece on its side as I closed the left-hand side of the structure. This had trapped the white pin inserted at step 65 (and pushed home at step 67) on the wrong side of the red L-shaped piece added at step 86. This jams the mechanism. So: keep the "T" T-shaped, ie with the stem pointing down, and the bar horizontal, when assembling. Hope this helps ;)