id have to say bad needle, also. a bur or bent needle tip, can always be an issue. its hard to tell, all bodies/skin are different also. ive done matching tattoos and had similar results, in the same area. one goes in like it was meant to be, the other one just takes twice as long and looks half as good. there are a lot of variables that can cause this though. skin type/pores/elasticity, bad grouping/needle bur, etc. you did the right thing though, have her come back and give it a once over. you can always try changing needles mid tattoo also.