The plums will float for a while but will eventually sink on the bottom after a few weeks!
According to my last year job. It would be best to add sugar later, 1 or 2 weeks , as it allows 清酒 get into the plum fully and will reduce the shrinkage and the plums will llook better.
It is normal that sugar stays at the bottom as it will take time for them to melt . Just need to give the glass container a few shakes for every week will do!