How to rev up the starter

It’s been about eight weeks since I hydrated the Breadtopia starter. I can use it to make acceptable bread if I double the amount of starter in the recipe and double the rise times. After feeding the starter, the volume increases about 20%, but it doesn’t double a some people report. How do I rev up the culture?
This is heel of my last loaf of bread and the new loaf, the no-knead recipe, 1.5 h into second rise.