The ability In games using this, at the start of your turn, gain a card from the trash costing up to 6 only activates if Shaman is in your hand. As I read it, it should activate every turn, even when you don't have any Shamans in your deck.
Edit: I got a Shaman game again and it worked properly now. Don't know what was happening earlier. Maybe I just did something wrong.
Thanks for the report.
I also tested again, and it seems to work. Usually it takes some time to get something into the trash.
If there are no other "at start of turn" effects, then it is silently ignored with an empty trash, or done without prompt if there is exactly one thing (or only one kind of thing, like 2 Estates) in the trash.
So it usually takes some time before the prompt actually comes up.
After testing a bit more, I think I know what happened the first time. I was trashing cards and not seeing any activation, because most turns I was trashing only one card per turn, they all ended up with my bot opponent (who wasn't trashing anything). By pure coincidence, the few times there was a card left over in the trash at the start of my turn, I had a Shaman in my hand, making me see a connection that wasn't there.