I have recently reached the conclusion that if closely marked, he lacks the speed over 5 - 6 yards which Tats (and also Wright) have to get away. The other clubs are not mugs, they will have done their homework. Blackburn let their guard slip, EMC put in two good crosses, but that was the exception.
Tats (and MVE) have responded to coaching and the crosses are coming in from the right in a way they were not under late-Robins. EMC made spectacular contributions as at Wrexham and Stoke because he has wonderful ball control, but is not doing the day-job - getting crosses in.
It's a speed over the ground thing, he seems unable to get away from his marker, or so it seems to me.