Why does this Biwire set-up not work?

I have to make some assumptions since your discription of what you are doing is very incomplete.
I assume you are trying to bi-wire a single pair of speakers that have dual input jacks on each speaker.
The problem may be just in the speakers crossover setup.
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One reason it does not sound right might be that your speakers using that method MUST have a separate crossover, external to the speaker for it to work. (My Infinities were ok to biwire, as is, because the crossovers took care of the problems anyway., but my Maggies need a special setup, to be able to do so. The Maggies crossover does not easily lend itself to bi-wiring, though it can be done)
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Another way this may be going down, is that the amp is trying to "see" two sets of speakers and is getting only one and thinks 'this is screwed up (low impedance) and the amp is self limiting it's output. (But that would be assuming your amp is pretty smart)
Have you stuck the two sets out wires on one only set of jacks from the amp, to the dual sets on the speakers? That would eliminate my "amp" worrying/thinking theory.
After that... who knows

What do you mean by "not right?"

how much time have you given the cable to break in ?

expect 200-300 hours

The issues is related with the internal rewiring from the binding post at the speakers to how it links inside the crossover board of the speakers you are using. While having the jumpers removed and thinking that a set of cables from the amp goes to the color associated with it, the problem may be that the second pair,while innocently doing the same thing to matching the color to the posts...a phasing problem occurs with the speakers, why? Some crossovers have their hi frequencies inverted by the design it was meant to be. The inversion is in the wiring that you don't get to see and not associate unless you look at the board.

I answered eairler in this thread that when I moved my right channel speaker cables to the second set of amp output terminals the sound got out of focus with no soundstage. After a second look at what I had done I noticed that the top set of terminals have the positive (red) on top and the bottom set has the positive on the bottom terminal, that is where I made my mistake. Is it possible you hooked one set of wires out of phase?