That is a good question. I am a fan of rebuilds - to a point. I think there is a right way and a wrong way to do it. But it is hard to pin down where they go wrong and what the right way is.
One mistake that I think I see is a lack of patience. As long as you are continuing to draft and develop superior players you are moving in the right direction. When you start trying to speed up the process by trading high value futures for proven 'now' players you risk just reversing the process. You need to win those trades. lose a couple, or even just break even and you end up having to restart the rebuild.
But I'm not certain that that explains a lot. It might just be anecdotal observation that wouldn't hold up to careful examination. Luck and timing also enter into it. Sabres have been guilty of losing patience and have had bad luck and bad timing.
Are they finally just a goalie away? Or are they just a collection of pretty good players but not a good team?