Nipple tattoo

I have had my plastic surgeon's tattoo artist work on tattooing a nipple for me to "match" the exisiting one on the other side.  Unfortunately, I have been through 4 tattoos that have basically not taken.  I am at a loss at where to go next.  I live in the Los Angeles area and I don't know where else to go.  The tattooing was first tried before radiation therapy - it did not take.  I have had three post-radiation and I have a little staining but nothing "to write home about".  This is quite demoralizing and I don't know what to do.  The tattoo artist does not use "metals" and I am wondering if this is why they don't take.  Does anyone have any suggestions that might work?