Or is it all psychological? A professional trumpet player said to me yesterday that after practice, within reason, your 'sound' and ability will come through on whichever mouthpiece you choose (note, not ANY mouthpiece - just the one that you choose for your 'chops'). Imagining a scenario, one can pick up a mouthpiece and after a few minutes , think it's the best thing since sliced bread. Two weeks later it all goes t*ts up. Is it the wrong mouthpiece? fault in technique? (more likely the latter as it's probably that that's convinced us that we need to change mouthpieces in the first place, perhaps? ;-)).
Perhaps, as I said earlier, it's nigh on impossible to deal with written 'agony aunt' style mouthpiece problems as maybe the answer lies in a qualified teacher to 'see' the problem and hopefully the solution.
For the record, I use three mouthpieces, depending on differing circumstances, a Wick 4B (for sop work) and a Warburton 3BC with an 11 backbore otherwise for cornet/trumpet and a Wick 4FL for flugel. I'm not saying that's right for anyone else, time may tell me that it's wrong for me, who knows, but I feel comfortable with it. I also have an 'off centre' embouchure! ;-)