With my GFs A1200 I had to use considerable force. I first thought the unit was properly seated but it wasn't (and I couldn't see a gap by visually inspecting the situation). I had to push it hard enough to bend the motherboard slightly.. when it finally locked in place it gave quote noticeable snapping sound. YMMV of course.
Remember 560SL that you can always take the motherboard out and support it from behind if bending it seems too scary. :nervous:
