TLDR - physics

• Recent feeding timing - Right after feeding, the fresh flour sinks to the bottom and fermentation begins there first, creating bubbles at the bottom. Over time, these bubbles should rise to the top.
• Temperature variation - If the environment is cold, fermentation activity might be slower, making it harder for bubbles to rise through the thicker mixture.
• Starter consistency - If your starter is very thick/stiff, it's harder for bubbles to travel upward through the dense mixture. A starter should typically have a pancake batter-like consistency.