Hi Shadowradex, i'm by no means an official game support person, but hopefully I can help. There are a few things to check here.
For offline: If you have offline damage prevention enabled I believe it may stop your Dung beetles from producing fert.
The official mechanics for the beetle should be as follows:
2 Small Animal Feces = 1 Fertilizer and 2 Oil
1 Medium Animal Feces = 2 Fertilizer and 4 Oil
1 Large Animal Feces = 3 Fertilizer and 6 Oil
Assuming feces is manually placed in the inventory it should make fertilizer and oil no problem. I've also seen where not having enough room for the beetle to wander sufficiently can sometimes cause them not to convert, but I believe that's more of a glitch than an solid game mechanic. In your case being that you're offline, I would check that offline damage prevention mode first.