May not have enough affection with her due to the choices you made when you first met her. Every payment day you can raise her affection by picking the option that compliments her. (note this hasn't been confirmed yet but its highly likely to be true. I will confirm if it is this weekend if you want to wait.)
Yes, looks like you can only gain affection the first payment day. I revised the walkthrough to include day 2 so people don't have trouble getting her hair later. So now it has all the times you can get her affection. You don't seem to be able to get affection from her anywhere after you miss these chances so you will have to start a new game.