You need to be loading those stones to a much higher platform man. You for sure have the strength to do it, you just need some more technique work.
When you are grabbing the stone, your hands are way too far fwd, which is why when you go to pick it up, it rolls out of your arms. Arms straight down, feet not far away.
When you have the stone in your lap, it should be just under or touching your chin, so that when you stand up it can explode out of your arms and onto a much taller platform. I'm surprised that you don't have some bicep/elbow/shoulder problems the way that you are loading stones.