The F2S has been my primary body for a couple of years. I'm not sure it makes sense that your DP2 works with some lenses but not others. More information or some photos of the lens and prism might help. Does the coupling prong on the 135 look bent or off-center? Are the lens or prism recent aquisitions? Is the DP2 seated properly on the body? Try removing it and reattaching it. Finally, it is just the window on the front of the prism that reads 5.6, or both windows? You might try emailing Sover Wong; he's an absolutely invaluble resource for F2 owners.