The assembly pin is retained by the assembly pin spring. Very possibly it has been bent or damaged during reassembly. It is shaped like a little hair pin and the legs fit into two grooves in the frame. Look at the diagram in your manual. I think it is part# 284 or 285, if memory serves me. You may be able to tweak it back. When the pin is inserted you should hear a click as it snaps into place on the pin. A new one is not expensive, and easy to replace if the old one is beyond saving.