You might have added too much milk or liquid if your chia pudding is runny. To fix this, whisk in another 1-2 teaspoons of chia, and allow it to chill for another hour to thicken. Besides that, your chia pudding might need to be soaked longer. You should allow oatmeal chia pudding to sit for at least 2-3 hours. This gives sufficient time for the chia seeds to absorb the liquid and thicken.  Lastly, you might have used chia seeds that had gone stale, which will not gel up properly.