I agree, Deb. It has to be one of two things:
1) the jib is allowed to back-fill too long (as soon as it helps push the bows across the wind, slack that sheet), or, more likely,
2) you''re assuming that centering the tiller once the main sets is sufficient to start off on your new point of sail. But there is always a slight delay, esp. in light air when boatspeed is low, when the keel "digs in," and you begin to accelerate on your new tack. During that moment, the bow can travel a great deal. In my 22ft. boat, pushing the tiller away (downwind) immediately after the main fills ensures that it''s the stern that will slip downwind instead of the bows, preserving my new heading. It''s normal, esp. in a small boat in light air or when you lose your boatspeed through your tack. (Remember, you have quite a bit less friction between keel and water than you do between tire and asphalt, and this is the moment of least fluid pressure on the keel. There is going to be some slip). It''s normal: keep practicing!