Drugs don't work on me?

I've done MDMA a few times, and cocaine once. On MDMA I felt absolutely nothing each time. After a line of coke, I felt a bit wobbly but that's it. (I also was barely affected by LSD, but I think that was just shitty acid.) I was with other people each time and they were all high, so I trust the sources. My current theory is that my antidepressants (Prozac and Wellbutrin) screw with my brain enough that it doesn't register the powder I just put up my nose. Thoughts? Does anyone else have this problem?