sometimes AutoPano-SIFT is fantastic, sometimes it makes a right hash of things.
When it fails, try going into the manual control point panel, ordering the control points by missmatch distance and deleting the worst few. Its often obvious that one or two of the worst fits are actually total rubbish and not the same point in the picture at all.
This is one of the things I like about Hugin, (these days) its fully automatic when you ask it to be, but when the automatics mess up you can take manual control as much or as little as you need to. Bit like a DSLR really