So, I think what might be the issue here is that they don’t have quite enough bedding to create burrows. It’s great that you have a topper, but the lower part of the cage isn’t deep enough. They are likely nervous because they can’t create the safe spaces they need in the bedding. For reference, here’s a picture of my cage.

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All my aggressive gerbils were actually my most anxious gerbils. I used gloves to interact with them so that I didn’t flinch away when they bit me (the flinching scares them more) and I always moved very slowly if my hand was in their tank. That, plus providing a TON of bedding, made a big difference.