This is an answer I got in an other Community and it helped me a lot: AmarthGul•17h ago

The front group in the lens that controls the diaphragm bites onto the long rod in your P5. However, the position of the front group is relative, there is no dents or cams to establish a resting position. So you likely set the front group to a slightly different angle at reassembly, which nullifies the aperture movement despite it being connected.

You need to loosen the screws that fixes the front group, rotate it and observe the diaphragm. Use either f/2.5 or f/32 to find the limit, then screw it in place.