It's so frustrating, isn't it? This is one of the few illnesses with which there is no one size fits all treatment.
Yes, it's entirely possible that the Committ is keeping the SAM-e from working. I don't know much about either substance, but your experience seems to indicate that it's a possibility.
You also might talk to your doctor about what is called a drug cocktail, or combination of antidepressants. Sometimes that's more effective than just one medication.
Also, talk therapy and antidepressants combined is the most effective depression treatment, so you might want to look into talk therapy.