PUL stink

[Reagansmama]

So I have 2 Flip covers that I've had for 2 years, and for the past year or so used primarily at night time with the flip cotton prefolds (works great btw - totally bulletproof for my 2.5 yr old and 7 mo old). However, one of them now has permanent stink, even after coming out of the wash! It's quite upsetting. I feel like the other one might be close. Is there any way to get this stink out? Or am I stuck? I use RnG and have for about the past 9 months, prior that that I used Free and Clear Ultra Purex. Any ideas?