I had the same problem. I figured it was because I added to much water ? I cut back on the water and its been fine. When I used a 5 gallon bucket I just added water and stirred it and then added more later after the first was absorbed. Seemed to work better that way without over flowing. I dont have holes all the way to the top but about half way & on the bottomSo my new buckets are overflowing from the bottom bucket out where the two buckets sit together. I didn't have the chick bucket more than half full, but the big chickens need a full bucket, as I go through one bucket a day (at least). I didn't put holes all the way to the top. Do you think that is why it's overflowing?
I still have two buckets to drill. I'm going to do the holes all the way to the top to see if that helps.