It's not arguable that Aquilani, Melo and Krasic were excellent, even brilliant, in one part of the season. What's strange is how all three of them suddenly stop performing. They lost confidence as Jack said.
Melo was a beast when things worked well, then he suddenly lost it and he doesn't have that confident look in his eyes, nor is he performing with confidence. Aquilani simply stopped giving a damn while Krasic is getting more and more frustrated (and more and more frustrating to us) when he sees that nothing works for him.
Your best three players to stop performing and to drop in form at the very same moment opens up lots of questions.