I’ve noticed that in 2.1.2, selecting a portion of a track and dragging the mouse all the way to the right in an attempt to select more will not make Audacity scroll to the right to allow you to do so, although doing the same thing by dragging the mouse to the left works just fine. I’ve never had this problem before.
The workaround could be scroll right with Shift-ScrollWheel (or shift-swipe) and then Shift-Click to add that portion to your older selection.
Audacity 2.1.2 has a number of little oddities like that.
Koz