The Seanchan have another problem attacking the Black Tower and that is hard intel on its layout. The WT can thank the Dark one for loosning the pattern enough for the novice quarters to be swapped around with sisters because before that happened the Seanchan would have a good internal map of the White Tower from captured Aes Sedai. They would know the layout and who stays were and what the normal night time routine is.

As far as I know they have never captured an Asha'man to get him to draw a map of the Black Tower. Why would this be important? When Taim's palace looks important but that is not where most of the Asha'man are. Likewise the Asha'man are not stuck in a narrow vertical structure, they are spread out and if you can't bring overwhelming force you are going to have to target where you strike. What happens to the Seanchan if they start blowing up the buildings that actually houses say the men and women who are servant's/craftsmen and women. Worse what if it is some of the Asha'man's wives and children but their husbands/fathers are in another barricks. You will have a lot of Asha'man popping up and start blasting everything they see in strange armour or grey dresses.

Even if they have managed to get some spies to report back the Black Tower is growing and changing. Any report that does not get back to the Seanchan quickly could be out of date again making an attack very hard.