Try getting an electric toothbrush. A good one with replaceable brushes, and rechargeable base.
She may be excited to use something new and different...trying it herself. I got one because it's so hard to brush well and long enough, and the electrics do so much more - faster and better than my hand wiggling. My kids love going to the dentist but I make them use the electric a couple times a week at least.
My kids are sometimes terrible about brushing their teeth at all, and the front teeth will turn yellow or orange at the gum line and be so gross. I brush for them when they haven't been doing it...whether they like it or not...no matter how old. (I have five of them, 10 through 2 years old).
Maybe you can offer a reward as well. Tell her about the reward first. Watch her brush her own teeth and then help if she isn't doing those back ones well, just help and let her keep the brush in her own hand. When she is done, give her some kind of reward and tell her how happy you are that she is learning to brush better.
Does she go for teeth cleanings to the dentist? Tell her if she cleans her teeth really good at home, then the dentist won't have so much work to do and she will be happy that she doesn't need fillings from getting cavities too.