constrain poles of Studio Spline

As shown below, at first I constrain two poles of "Purple" to the ends of "BLUE_1". When I try to drag the third pole to the end of "BLUE_2", it's impossible to be done! It seems that I need to set Movement to X/Y/Z and drag the pole to the target point for 3 times and finally get them coincident.

 

I never had this trouble before. Do I miss something important for Studio Spline?

 

Thanks!

 

ADD: I just find the solution --- set Movement to View.