Isoparms Problem
 

I'm working with nurbs sphere as a modeling techniqe as what this video teaches, and I love it . the problem that i'm having is that it was working out great in what i was modeling. Now i'm having trouble into a fouth step which i need another nurb sphere, i had to insert another isoparm and when i do that, and hit insert isoparm i can't select or it won't high light the section that i need to delete, which is the top of the sphere . I'm not sure if i hit a certain key, tool, function that i wasn't supposed that change the way it works. Am i supposed to change my nurbs first into polygons . I'm lost any suggestions would be great. Thank you