I realize that this thread is 4 years old (!) ... I came across it because I ran into the same situation of not getting a leader for symbolic independent labels.
GS support kindly pointed me to the checkbox shown in the attached screenshot, which I had long forgotten existed. If there is no check for 'Use symbol arrow', then the Label Tool will draw the leader. If it is checked, then there will only be a leader/arrow if one is scripted as part of the selected library part.
Getting old,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB