I'm in college taking an intro class in aerospace and we have an assignment in which we are to build an existing air or space vehicle. I picked the space shuttle and proceeded to make it. Only being my second semester using CATIA, I don't know a whole lot, but I do know the basics. I've gotten the front selected and created a multi-section solid, but, for reasons unknown to me, it won't let me select the last two sketches to complete the solid. I'll click the multi-selection solid button but when it is time to choose the sketches I want, I am unable to do so. When I pass my cursor over the sketches in the tree (sketches 13 and 17) the cursor changes to a black circle with a horizontal line through it. Any suggestions? thanks