Well, to me this could be down to three things.
Firstly, the obvious one, Cordy was no longer there to pass on the visions to Angel, therefore they may just not have been able to contact him. Clearly there had to be a conduit or what would have been the point of putting Doyle or Cordy through the agony of the visions.
Secondly, they may just have decided that Angel was no longer the champion that they believed him to be. He had agreed to work with the Senior Partners and so his might have devalued him in their eyes.
Thirdly, and the one i tend to really believe, Angel was exactly where the PTB wanted him to be. They chose Angel as their champion for a reason and they must have been aware that he was important to the Senior Partners as well. I reckon that they had always planned to position him at Wolfram and Hart and let things run the way that they finally did because they knew that ultimately Angel would do what they wanted and bring the firm down.
Now whether that meant they were willing to ultimately sacrifice Angel and his team or whether they planned to step in with some form of help that night in the alley, who knows? I have always believed that if Angel and co. did survive the end of Not Fade Away then it was because he had help from the higher powers.