Here is a method I have found that will usually get those stubborn buckets apart.
Fill a larger container with warm/hot water. This container must be large enough to sit the buckets into.
Next, fill the top bucket with cold water. the colder the better!
Now, sit both buckets into the container with the hot water. You want just the bottom bucket in the hot water.
Wait a few minutes and then try to pull the top bucket up and out of the bottom one. Usually, the expansion of the lower bucket by the hot water and the contraction of the upper bucket filled with cold will allow you to get them separated.