Not letting it double. I was impatient and kept refreshing it even if i didn’t see any (or very little) activity. I had to wait longer.
Put it in the fridge too early. I put it in the fridge on day 7, while there was still no activity (probably due to point one). I had to put it back to ambient temperature to fix it.
Little oxygen available. I was putting a lid on top of the container. While it was just resting there unscrewed, I think that it reduced the amount of air going inside the container too much. I fixed it by replacing the lid with a kitchen towel secured with an elastic.
Too much water. I was hydrating it too much. While 100-110% hydration should be ok, it seems to grow much stronger now that i reduced it to 90%.
Changed flour too early. Changing flour is ok, but i did it one of the first days of the starter. I think that might have messed up things as I went from a 100% rye to a mix of whole wheat and strong white flour.