The picks and screens have not been as effective. Ray is not getting the same amount of space.
In the NBA, it is rare to get much space. Ray just needed that little. That little is even smaller now.
I think its a combination of the picks and screens being slightly less effective (although I don't think this has been as bad as some suggest), Rays legs are just a little slower (from fatigue, but also the mental fatigue of being at this point in the season), and defenses know exactly what's coming.
I really think the best way to get Ray back into it, is to stop looking for him. Currently, they are zeroing in on him, like Brady would on Moss, and defenses are just taking it away. In order for it to open back up, they are going to need to make a concerted effort to run some different plays.