Oh I am so glad I found this forum! I was starting to have this exact same thing happen with my Kefir. A dear friend gave me some of her very healthy grains and they did well with me for the first week. The second week they were starting to seperate way too fast and my kefir was getting very runny, yet curdled at the same time. For some reason, I did not even think about the fact that I was letting them get way too large and yet using the same amount of milk.
So I culled them way back and have now only a couple tablespoons in 2 cups of milk. Now I'm going to check every few hours, give them a swirl, and change if they start getting too seperated.
My question then is this: While I am learning the ratio of grains to milk, is it alright for me to change them in less than 24 hours if they look like they need it? Then I would need to increase my milk or decrease the grains until they are doing their thing in a 24 hour period correct?