[Tom Wesenberg]

Are you sure they are stipped? Check to see if the bolt is also turning when you turn the nut. If it is then you need to push up so the slot in the bolt head engages the cross pin.

This will also mean you may have to make a spacer to fit under the bolt head to lift the bolt high enough to engage the cross pin at all times. The bolt pockets get worn from vibration and the bolt drops lower.