If she has not gotten any positive relief from the Zoloft, there are many other SSRIs that could be tried. Also, Wellbutrin may be a good choice, as it combats depression as well as smoking urges.
Is your mother seeing a psychiatrist as well as a therapist? Of course it's possible that you mother many not be suffering from OCD, but a related anxiety disorder or from a bi-polar disorder.
Good Luck.