There could be several different reasons the pudding is too thin.  If your chia seeds are not fresh and don’t gel well.  Some brands thicken better than others. The oat milk or other dairy milk was not very creamy.  The pudding will thicken after two hours, but it’s best to let thicken longer or overnight.  If you enjoy a thicker pudding, you may need to experiment with the chia-to-milk ratio.  To fix the pudding being too thin, add more chia seeds and let it thicken for about 30 minutes to an hour more.