can't get coloured ink to take!! help!!

hey i've been training for a few months now, no problem with black and grey, but for some reason i cant seem to get coloured ink to take, i've set my machine and needle the same way i would as if i was shadin. volts i set around 8. the inks are ok themselves, i don't know if im working too fast , too deep or not deep enuff? i was getting trained by my tattooist, who just so happens to be doin time right now.... oh the joys, which now means im havin to try and teach myself. lucky for me i have a few people that a willing to let me use them, as i said black and grey's not an issue, so if anyone could help me i'd be ever so gratefull. thanks tracey

what ink you use????