Well if she sat for 2 years, she may lack the proper muscle to hold a lope for long. And she's probably lazy. It's not easy being a pasture puff for 2 years and then going back into full time work
My advice, give her time. You've only had her a week. Keep getting the walk and trot down, and build her muscle and tolerance. After you have that down, then you can worry about the lope.
How old is this mare? Do you use a lunge whip when lunging?