First and foremost, verify that the Modelling Relation, if there is, is frozen. You can do that with Model > Effect > Freeze > Modelling Relation.
When there is an active Modelling Relation, it doesn't matter what changes you made, because the instant you move the Timeslider the object will revert to its reference shape.
I know 2 ways to cap a NURBS object.
1-Go into Info>Selection, and check Capping Top/Bottom.
2-You can try to extract your splines, then make a skin with these. When the dialog box pops up, check Polygon, then check Cap, then finally check NURBS (so the Cap option remains checked), press OK.
If there is other ways, please tell me!
By the way, what is the Quickselection feature?
Bernard Lebel