What do you mean it is not auto correcting?
It is probably because you have either added the words to dictionary or kept making typos

(it learns your habits, keep doing something frequently and it won't auto correct that miss spelt word as it will start to think it was intentional, - to reset all words added to the dictionary manually or automatically, go to keyboard under settings --> click on advance --> reset suggestions). When I first got my phone, one of first things I did was test out the keyboard I deliberately made typos constantly and after awhile it stopped auto correcting those typos.
I have been using Wp8 for almost two years and I can tell you hands down word flow is the best one out of everything I have used (I have used iphones, windows mobile phones, android, blackberry physical & touch keyboards). Plus it is aggressive (but not overly aggressive) and context sensitive. Shape writing is the icing on the cake, this is similar to swype on android.