The R19, a medium-performance keelboat, is not at its best in very light air.
But put simply, many boats won't point, or tack, under jib alone, though they'll reach and run all day. So your experience is typical. Maybe you could get her to tack with more wind, reach-to-reach, but not worth trying, just jibe instead. It's easy to bear off, hard to point up.
You need more sail aft of the boat's pivot point (which is about mid-keel), to "balance" the boat, and hence the tiller, for a smooth, controlled tack. That, of course, would be the mainsail.
With main alone, it would be easy to head up (though you'd be very slow without the jib to accentuate the airflow along the main), and quite hard to bear off, epecially in a good breeze.
That's why they designed her with two sails. Boats with only one sail typically have the mast farther forward to get that "balance" between center of lateral resistance (keel) and center of effort (sails).