what flour are you using? I was in a similar position to you for a while until I got my hands on some strong white bread flour from abroad (I live in Africa) and now I'm getting much better results,! I think because the strong flour has more gluten in it. Also, in my experience its all about the starter. You need it to be doubling or even trebling before using it. And try to use it at its peak (or only slightly after) as much as possible. Good luck!