Those straddles definitely look too long, the arms on these are quite long and "vertical" which makes the problem worse (did campag make "narrow profile" cantis? I'm not sure but these definitely look "narrow"). With the setup you have now, there'll be little mechanical advantage when blocks are touching the rim so that's why they don't work. Plus old pads sometimes aren't the best, they harden and dry out, then don't grip the rims. one thing or the other and you might be OK, but both these things together and it's no wonder they don't work! Try shortening the straddles until the hanger is aprox half the distance from the tyre that it is now, and see how they work. Then swap for decent modern pads and see if there's any more improvement.
There's a good explanation on sheldon: https://www.sheldonbrown.com/cantilever-geometry.html
There's a good explanation on sheldon: https://www.sheldonbrown.com/cantilever-geometry.html
