A clump of cells will levitate to its average density, even if various cell types (i.e., dead cells stuck to live cells) are contained within the clump.
To avoid clumps, there are many common additives used to reduce cell-cell interactions, such as EDTA and DNaseI, which are compatible with the LeviCell.
Decreasing cell concentration can also help with clumping as the cells are no longer in such close proximity.
Consider passing cells through a filter to break up clumps.