With some lenses, when you remove the prong and replace the screws, the screws are too long and contact the surface inside the aperture ring. If it barely touches there will be increased and uneven resistance on the aperture ring, but more likely the ring will lock up completely. In this case either remove the screws, or shorten them, or source some of the short black screws that Akira mentioned (these screws seem specific for lenses with the prong removed, they are not from old pre-AI lenses)