There can be a few reasons for this. 1. Your starter isn’t established or mature enough (it’s on the younger side) and doesn’t have the strength to give your bread the rise it needs. 2. You’re pulling from your starter too late (it’s started to fall when you use it) 3. Your dough is getting over-proofed or under-proofed which can cause dense bread. 4. You aren’t using enough hydration (water) in your dough. 5. You might not be baking at a high enough temperature, especially if you have an electric oven.