Three possibilities come to mind:
1. The scheme works in theory but not in practice. Or if you want to get cute: it works in practice but not in actual games. For instance, maybe it is too complicated for college players. It needs 11 guys who fully understand the defense and can execute it without hesitation. But instead, there are only a handful of guys who have reached that point. The rest are hesitant, are thinking too much, and this in turn leads to a breakdown in fundamentals such as good tackling.
2. The scheme should work but the coaches have failed to properly teach the scheme to the players.
3. The coaches have failed to bring in the right players to properly execute the defense.
Whatever the case, after 3+ years it falls on the coaches. If it hasn't worked yet, why should we believe it will work next week or next month or next year? There is always going to be a mix of experienced and inexperienced players, and there will always be deficiencies somewhere with regards to personnel that a coordinator will have to find ways to work around. Coaches that fail to adjust are coaches that will fail.