Below is some correspondence with a gentleman who was having trouble healing his second set of nipple piercings:

Dear Elayne,
First off, I want to thank you for all the things you've done for the piercing and body modification community. You are truly a pioneer and have helped so many of us along our journeys with your work and writing. 
I have been having some trouble with a newer nipple piercing and have been unable to resolve the problems I've run into, so I figured that I would turn to the most knowledgable person I know of when it comes to piercings, you.

About two months ago I got my nipples pierced a second time, I already had a set of well healed and toughened horizontals for years.  The piercings were done complication free, tucked right behind the existing horizontals at the base of the nipple, possibly ever so slightly into the areola.  They began to heal relatively problem free and I commenced with my usual aftercare of daily Provon washes and twice daily sea salt soaks. 

But several weeks in I bumped the right one pretty hard at work and I developed a lump behind my right areola behind the new piercing. Initially it was weepier than usual like but didnt seem at all infected, no green or foul discharge, no pain or warmth, just a hard and somewhat tender section of tissue about 1cm around.  After doing some research I have come to the conclusion that the lump is the lactoferious sinus as it feels like it conforms to that shape/location.  I went to see my GP just to be safe and he said he thought it was inflimmation due to the piercing trauma and possibly bumping it or trauma from sleeping on it. 

He sent me home with instructions to continue my sea salt/cleaning aftercare and to be extra gentle with it.  After increased saline soaks plus an additional hot water soak for several weeks it calmed down but never fully went away.  And just the other day I noticed that the lump had grown again, with no real catalyst.  It is still there, is somewhat tender, and Im wondering what you think could be going on.  Thanks for your time, Z.

Thanks for the positive feedback; I really appreciate it.
Sorry to hear about your trouble healing your nipple piercings. There are a number of things you can try. I would highly suggest keeping up with the saline soaks.
For piercing care and troubleshooting, I’ve found something immeasurably superior to anything I’ve ever suggested previously. I am advocating the use of Briotech. You can order it right from that page of this site.

I’ve done a great deal of research on it, and it is absolutely incredible! The active (and, other than saline, only) ingredient is the same substance produced by our white blood cells as part of our immune system. It reduces inflammation, destroys pathogens, and fosters healing! It is the first aftercare product that has been used for piercings that has numerous scientific studies proving its safety and effectiveness for wound healing. Links are on the Briotech page. Further, it is far more effective and efficient at destroying germs than anything else, (it has a 30-second kill time for MRSA!) and it does so without drying the skin or harming tissue. All you do is spray it on, so it is the easiest aftercare as well. It is so bio-friendly it is safely used in the eyes, nose, and mouth too!

You can see photos of my own near-miraculous healing experience with it here.

This idea may surprise you, but some people have had success with a natural remedy that is used for nursing mothers: cabbage leaves.
"Strip the large vein from the cabbage leaf and cut a hole for your nipple. Wash (to remove any chemicals) and chill before use. Wear a leaf inside your bra. Change every 2 hours. Stop applying the cabbage leaves once you notice the swelling is beginning to lessen." You might try applying it to the area using a wind or two of an elastic bandage around your chest to snug it on.
Some piercing clients with irritated nipples have had good results with this.
Unfortunately, they changed the formula of Provon and added Triclosan. Therefore, I'm no longer suggesting it. I
If you want to use a soap, you can try Dr. Bronner's Baby Mild soap (pre-diluted 15:1 with bottled or distilled water). It is MUCH milder than any of the other soaps, which is preferable to avoid killing any of the delicate healing tissue.
Give a try to whatever seems appealing to you, and let me know how it goes.
Sincerely,
Elayne

He sent a follow up six months later:

Dear Elayne,
I wrote you back in February about problems with my 2nd set of nipple piercings.  I wanted to let you know that it took months of ups and downs to get them settled but they're healed nicely now.  It took trial and error but I eventually found that they were healthiest if I only wore shirts for a day or two max, showered and cleaned the piercings with Dr. Bronner's soap Lavender and Baby-Mild) were the best for healing. I followed that with a drop of 100% tea tree oil worked into the piercings every day, and totally not touching them except when cleaning. 
It worked out well, one definitely took 2 months longer than the other. Thanks for your advice before, the cabbage leaf was really helpful in getting the irritation down. 
Since I last wrote I started stretching my ears again (they are punched at 4g) and am back at 00 which is great, I missed having dangly lobes. Thank you for your advice before, hope all is well.