Hi Majik, unfortunately, the left click for use does not work, so you can roll the snow balls but then can't stack them or add anything. Played on PC, if that helps.
Are you sure the ball is big enough? A subtitle is shown once it's the right size. If it was, did you leave the big ball near the middle? If you didn't, there's another subtitle hinting to go closer to the middle.
Just checked and all the interactions work as intended. Stacking is done by rolling the small ball into the big ball btw.