How much heelwork are you asking for?
You might just be asking for too much.
If I were you I would break it right back down to basics and pretty much re-teach her how to heel.
To start with reward her for just being in heel position. Then start adding one step. Reward in position, break and play. Ask for one step again. Reward, break and play. Repeat until she is excited to take one step with you.
Now ask for two steps. Reward in position, break and play. Two steps again, but if the first step is perfect and awesome then reward just the first step instead of asking for the second to encourage that awesome work.
Once she is doing two steps really nicely with nice position and focus ask for three steps in a row.
Take it really slow, stop if she loses focus. Don't beg her to focus on you, just walk away.
Do not focus on getting long stretches of heelwork at this stage, do not add any turns. Three good steps is better than ten bad ones.
Work on turns seperately. Once she is able to do left and right turns nicely you can start to slowly create a sequence of three steps then turn, reward and party.
Once she can do three steps and one turn nicely with good focus you can add three more steps of heelwork (three steps - turn - three steps).
From there slowly build up but avoid long stretches of heelwork, focus on enthusiasn and position. Duration will come later.
I hope that helps
PS: watch how you position the reward, keep your hand on your leg so that she has to press her face close to you to get it.
Also, taking a quick side step can liven up heelwork, keep the dog paying attention and encourage them to get closer (opposition reflex).
