Sourdough starter doubling too slow
I am quite new to bread baking. I have recently started my own starter with whole wheat. I've made a few loafs of bread with it and the results have been good. I usually feed it twice with white flour before using to bake bread and refresh it with whole wheat again. The starter usually doubles in about 6 hours, smells nice, and have a stringy texture.
The problem comes this weekend when I wanted to bake. I took the starter out from fridge, fed it, it didn't quite double after 24 hours. I wasn't quite sure how much volume I had to start with to determine exactly if it has doubled or not, so Idecided to feed it again with white flour. Since then, it has been working very slowly. It still doubles, but it takes at least 12 hours. And when I stir it at the end of the 12 hours, it's more like a soup rather than stringy.
I don't know what is going on. Could it be because a week ago when I fed the starter with whole wheat, I didn't wait for it to double and put it straight into fridge? I have tried googling but can't seem tofind explanationoff why it is doubling so slow. I have also tried to reviveshe starter with instruction from sourdough home. Still noimprovement.
Can someone help me please?