Hair wont grow past thins point?

I wear my hair in flat twists or cornrows most of the time, never out; I style it whilst wet like like this, using a leave-in conditoner and oil for slip. I get knotless braids once or twice a year. I wash it once every 7-10 days and oil my scalp and ends. I don't blow-dry or straighten. (Pic is freshly washed hair, haven't conditioned or anything yet.)

I think I'm having problems with length retention, I'm not really sure what to do or how to figure out if my hair is healthy. Any advice on maintenance would be helpful xxx