It sounds to me like the coarse grain needs something like pre-added clay, dust, concrete, etc. into the material itself to make it more dense than water, and carry in permanent plugging substances. It likely gets air trapped in the grains and wont sink.
That stuff looks exactly like the material gardeners add to hanging flower arrangement soils to cut the amount of watering necessary. It is hard to make it sink.
What is cool is my son and I picked up a set of guns that use polymer balls a little larger than Airsoft pellets when "inflated" with water. They show up in little packets of 1000 balls about the size of a pin head. Let them soak in water, and they are about 1/4 inch in diameter. This is about 20x or more size increase! Boy are they fun to mess with! They are all over the sidewalk outside now. When it is dry out, you don't see the bullets since they shrink back down. Once it rains or have heavy dew, all of a sudden there are bullets everywhere!
