I cannot get Crystals (or other stackable items) to stack in my Item inventory.
I'm using the + key, choosing Auto, then choosing Yes. It sorts everything together, and by name, but does not stack anything.
I've also gone into the Config->Gameplay menu to make sure Inventory is set to Sort (On) and Type is at Type 1, but I've also tried Type 2. I cannot get anything to stack.
Advice? Thanks.

Just to be sure, do you have groups of 12 of one type, or singles/ groups less than 12. They only stack to 12, not 99.

Same goes for clusters.

I currently have about 5 single Earth Crystals in my inventory. I'm trying to get them to stack. And as I collect more of other types, I'd like to be able to stack them as well in groups of 12.

Go to start > config > gameplay > inventory and set it to auto instead of manual.