I started to get some fog in my Crowbars during a -30 day in Quebec, it was because of my neck warmer. If you cover any vents then they start to fog. I had my neck warmer over the bottoms of the goggles, and that forced hot air up through the bottom vents, and it started fogging the bottom of the goggles. I just tucked the neck warmer up under the goggles and that pretty much eliminated it.
Just find out where the fog is coming from in the goggle, and then unblock the vent. Otherwise I love these goggles and they keep the fog out even in the shitiest conditions. Don't give up on them, cause unless they're defective, they shouldn't fog on you again.