A lot of new mamas like to have the security of people around. Maybe not messing with them all the time, but just being in their presence seems to help make them feel more secure. So, you might want to see if you can spend more time near her. And, make sure she is going back to the kittens frequently.When we left her, she was crying at the door for us.
I am sure you know to make sure none of them, including mama, can get too close to that heater - just feel compelled to mention it (sorry).