I think it needs a few days to adjust, at least at my airport it got better after a while.
But still is happening, in fact I’m facing exactly the same issue, but with large stands (not having small ones):
My medium stands are fully planned for the whole week.
My large stands are often not filled completely, even if there are large flights.
But As I’ve already mentioned here: Airplane size/percentage bar - #17 by sirhc80
I guess it has something to do with system not supplying enough large flights, as even the unplanned large flights by far wouldn’t be enough to fill my gaps in flight planner…