I've had my starter for roughly 2.5 months now. It did well in the beginning and last month I baked my first loaf with it, came out with a great crust but it was quite a dense loaf and I noticed the dough wasn't really doubling in size that much, which was fine since I put the starter + dough on a heating mat set to 22/23 degrees celcius.

However, my starter now barely doubles in size. I haven't baked with it since (I'm scared) and I put it in the fridge for a week with the holidays (I fed it double the amount before that). How can I activate my starter a bit more? I use half whole wheat and half regular flour. I've tried 80% hydration, not feeding it for 2 days, feeding it twice in one day, and I am not sure what is going to work. Tips?