I've seen this problem before and i think it's to do with the sear. If it's worn down then it could be quite hit or miss in terms of engaging. Or the recoil spring could be too powerful., which would mean that the bbs would feed, but the hammer would not go far enough back, which would fire again when the slide goes back forward. Again, it is usually quite hit or miss, so it might fire 3 or 4 bbs full auto before the sear engages. Check your sear, check your recoil spring assembly and you should find the problem.