it is impossible to change the automation curve when two points are all the way at the bottom. highlighting the points sets the ‘stretch’ cursor, but the curve span can neither be raised nor stretched.
to reproduce -
- open an animation lane and make sure animation for the lane is read-enabled
- click 4 points on the envelope
- drag the inner left point all the way to the bottom left and the inner right point all the way to the bottom right of the lane
- select the two lower points you just dragged, using the standard selection tool. that section of the lane will highlight and the cursor will change as it should when you do a range adjustment, but you can’t do anything with it, no matter how frustrated you get.