As living organisms, kefir grains are sensitive to many factors and sometimes, it takes shorter or longer to ferment. For example, kefir grains might need a little longer to ferment if they just came shipped or if the room temperature is colder as usual. Watery kefir can still be good and it still has all its probiotic benefits, but if your kefir seems too runny, you can let the milk ferment for a couple of hours longer. A trick that works very good is to use a smaller amount of milk: usually, the batch get thick faster. Be also aware that kefir ONLY thickens with dairy milk or high protein vegan milk (like “soy” milk).