I've been treating an abdominal rash that seem to be ringworm with OTC clotrimazole for three weeks, it doesn't seem to have changed much. What next?

Need to examine. It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. Since you did the appropriate treatment based on your judgement and it did not work, it would be prudent to see your doctor now.
Ringworm. Clotramizol is an antifungal cream, after 3 wks of treatment you should see some changes, it is time to see your doctor or dermatologist to make sure is it ringworm or not & to get the proper treatment, you most likely need a stronger prescription cream from your doctor . Good luck.