source controls a1 a2

Sometimes I don't see the source controls a1 and a2.

Right clicking on the destination controls (A1, A2, A3 ....) doesn't show a1, a2 or none.

Selecting a clip in the browser, and playing it in the Viewer brings the source controls back (but not always).

Is there a way to bring the source controls back?

Thx,

Marc