This weeks lesson went well but my instructor said the ginger one 'isn't balanced enough for canter' so how can I improve this? She will walk trot & canter on the lunge
The question should have been put to your instructor as they have passed an opinion they really should advise on what to do.
I would keep on doing some lunge work, practise transitions keeping the actual canter periods short, looking for quality rather than quantity.
Do the same when riding loads of walk trot, trot walk transitions, get her really moving forward from your aids in good quality balanced walk and trot, plenty of general suppling exercises to improve her balance before cantering her. Canter would probably be easier for her in a straight line, ideally uphill out on a hack to allow her to find her own balance, with you in a light seat staying off her back.
I have a 5yr old cob and its taken 3 months for the canter to finally click. I've done this not through lots of school work but slow walk and trot work in the school and lots of hacking, variety of canter pace in straight lines and letting him find his own feet. Hey presto, he can now balance! I think lunging is all very well but riders weight needs to be taken in to account.