Just thinking out loud here.
The past three managers have played very similar formations and tactics here right? moyes was more defensive , Martinez more attacking and koeman in the middle. But either way similar formations and largely similar players in those positions.
So is the simple answer change it? You read so many times of players these days being reinvented in positions, valencia and moses wing backs, milner left back, in changes of systems thst actually work and get results. So perhaps that is what is fundamentally wrong? the same players in the same formations and no chance of a change bar switching a winger from one side to the next