I've known quite a few horses who just aren't comfortable getting into a lope.
It would be a bit easier to give advice if we knew a bit more about her training (Is she just being started?).
One thing I like to do before I lope a horse is to have good control of the shoulders. I like them to be loose and the horse to know how to give them. Also how to bend, which allows them to know how to lift the ribcage which the makes the transition from a jog to the lope easier.
A great excerise is the leg yield. It teaches them to relax and give the shoulders while also flexing and bending and lifting their ribcage for more freeable movement.
Also good practice is the basis for spins and turns on the forehand.
The reason I like to do that first is it is a whole lot easier to get them ready to pick up their proper lead so you don't have lead problems furthur on in the training.
Are you having problems getting her into the lope while you're lunging, or just in the saddle?