Same here, I learned to do it with both side. Also try curling all of your toes except your big toe and vise versa (kinda like a thumbs up with your foot). Its one of those things everyone can do, but most dont because their body is pre-wired not to. Its just mind over.. well your body. Im convinced popping your pecs is also one of those mental things. Its helps to have muscles but I think everyone can do it.

ymmv though, I think. I taught myself as a kid as well, but I've never gotten very good at it. I don't have a lot of eyebrow inflection anyways so I think that's why. Some of us will never be Spock.