How long have you had her and how long has she been in work? I have an exracer and worked on the track, they actually work them on both leads and they are taught to swap in the stretch (differently then we ask though). So the fact that she is only holding the left lead may be a strength or a pain issue. Most horses are right or left "handed" and prefer one lead over the other. Have you tried going back to basic flatwork for a couple of days? Is something making her anxious? Most exracers like routine in daily life, get fed, turned out, brought in, at the same time, but bolting during racing season is something I haven't seen.