When fish have been in an area for a long time, they know where is safe and where the food is and whether there are predators to be on guard for Your fish have been in the tank long enough that they secure that there are no predators going to snatch them up so they have little reason to be more active. Tetras in general also are more active because they like to have a little current in their tank. This causes them to be more active or they get swept away with the current.
Truth be told, I wouldn't be concerned that they aren't swimming constantly and that they have a place they feel secure in. I don't believe adding more Tetras will change this. Changing them to a larger tank with more current would. As Aiken stated, a 10 gal
is really on the small side for this type of Tetra.