A peg that is parent to drawing elements that are inside a group will not have the collapse/expand arrow in the Timeline if the peg is outside of the same group. It will not be possible to visually collapse the peg in the Timeline. This only occurs when groups are not shown in the Timeline.
Ungroup the drawing elements then regroup them, this time with the parent peg included inside the group.
Mac OS X (all)
Animate Pro (all)