Another issue is that in the past, during the difficult phases of those tough games, they normally had midfielders or defenders scoring the important goals when the front three couldn’t. Now, all dry out in front of goals.The main problem is obviously the manager and their front three. The manager doesn't seem to have plan B while the front three are drying up, one or two of them need to be dropped or sold or replaced in the summer. I'm assuming it's part of football team/squad where you can't keep relying on the same front two (playing 442) or front three for way too long. The front three have been playing together for almost four years at their peak, sooner and later, opposition teams will or have figure out how to play against them. Also not easy to maintain consistency at your peak for so long. This is why Keane rated Giggs much higher than Liverpool's front three, to maintain the consistency in a top level is not easy.
Maybe there are also some mentality and attitude issues. Remember the reaction of Henderson in the post-match interview after he heard our players feel we should win. Arrogant and lack of respect. He might feel the results are granted for them?