It could be many things. I would say that if the dough isn't too wet (conversely if its too dry that also may stop it rising) then it is over-proofed. Try shaping the loaves then proving them in baskets in a fridge overnight. This slows the proving down and also helps the loaf from spreading too much when baking.
I also bake my loaves for 20 mins fully covered at 240 C and then 25 mins at 200 C. This helps with the oven spring and ensures the loaf is cooked through without burning the crust
It could be many things. I would say that if the dough isn't too wet (conversely if its too dry that also may stop it rising) then it is over-proofed. Try shaping the loaves then proving them in baskets in a fridge overnight. This slows the proving down and also helps the loaf from spreading too much when baking.
I also bake my loaves for 20 mins fully covered at 240 C and then 25 mins at 200 C. This helps with the oven spring and ensures the loaf is cooked through without burning the crust