Try changing your pace. Put her on the lead at Sit and then say Quick Quick and shoot off at your best speed so she is trotting/running along at the heel position. If she gets out of the heel position give her the Sit command immediately and you stop. Once she gets the hang of 'this is where I'm supposed to be' then you can go at normal walking pace. I would not use Heel as this sees to mean something different to her, just use Walk or Close or any other word you want.

Sgurr