haven't used mine in ages either, but i had a fair few color changes for a while. i would try to zap a two pets at a time so that when one of them seemed like only gender changes or nothing happened, i'd switch to the other for a bit.
The lab has always been random, being color changes the ones with less probabilities of happening. I saw a page some months ago about how high is each percentage of change and stats changes are the most common since you have stats raise and decreases for each one. So yeah, just a bad luck thing
I use my lab often and not too long ago I got a wraith zap