It's the hemp. If you can, try to use all cotton at night (as JenInSeattle mentioned) and see the difference in how it smells in the morning. It's not the hemp itself, it's that the hemp is dense and when sewn in layers, holds in waste and bacteria.
If you boil your hemp inserts for 10 minutes or use chlorine bleach in the last 5 min. of your wash on your hemp diapers, you will be shocked how fresh they smell in the morning. They also don't get as bad smelling in the diaper pail.
I'm switching to hemp flats to try to end this stinky hemp problem. I figure one layer of hemp fleece should get pretty clean without jumping through the extra hoops. If not, I'm done with hemp for regular daily/nightly use, cuz Caden's not a heavy wetter anymore.
Oh, and if you are one of the only ones that's never had stinky hemp, I either believe you have a different definition of stink (some say it's normal pee smell, not stink), or you use chlorine bleach from time to time, or you have hemp that is not very old, or you have very thin hemp (no more than 2 layer) items and have a great wash routine and line dry. I also think the natural oils in the hemp are what is "antimicrobial" about hemp, and when that gets stripped out of the hemp after a few weeks or months, it starts to stink. So if your hemp isn't too old, hang on a bit.