To the OP - don't give up! It can be a turd, but you'll get it!!!
I've been doing nothing but printing white on black for weeks.
Even took up printing white plastisol transfers.
I've been getting by, but I know this could be a lot easier. P/F/P/F/P seems excessive, but it's working, going fast, and yielding good results.
After watching a Bill Hood video on youtube, I was so p!$$ed off at my process and results that I have been going back to square one on troubleshooting - a new squeegee, a new screen, a new exposure unit, a conveyor dryer, step tests, and some new white ink.
Getting better
Gotta say tho, if I ever get one hit white on black prints like Bill was in his video, I'm gonna crap myself.

Here's hoping!!!
Bill- thanx for the vids by the way...
Any particular white ink would you suggest for on 100% cotton black ring spun printing???
I do a ton of this, and I really need to find a killer white for this. I am SO far from one hitting white on black...