Restoring "tooth"?

[Chuck Yokota]

When painting on top of a previous layer of paint that had dried, I have occasionally come across a a small area that has seemed to dry with a particularly smooth finish, so that the new paint doesn't adhere properly to the canvas. I will apply some paint on top, but when I try to smooth out my brush strokes, the new paint glides away, and the old color shows through. I am concerned not only about getting it to look right, but also whether the paint layers might separate in the future.

Is there a good way to restore the tooth on the surface of the painting? I've thought of sanding, or spraying a workable fixative, but I hope I can get some advice before I mess it up.

Thanks in advance for any help.