I do believe the mixture with Cannabis would be because Wellbutrin (Buproprion I believe the chemical name is) interacts with your Dopamine levels, which is also how the euphoria from cannabis is caused (indirectly).

If it's not working, you can try switching over to any medication that effects your seratonin (Normal SSRI's) or something more natural and less tested as 5-HTP (I use this for my Seasonal Affective Disorder).

But yes, go talk to your doctor.