Although you are dividing up 'p' so that each loop iteration works on a different piece, unfortunately you aren't doing it in the simple manner required for a PARFOR sliced variable. I think in this case because each iteration works on a differently sized 'slice' of 'p', you need to divide up 'p' to be a cell array. Something like this might work:
CellOccupations = round(rand(50, 1) * 20);
p = rand([sum(CellOccupations), 3]);
pSliced = mat2cell(p, CellOccupations);
parfor idx = 1:numel(pSliced)
pSlice = pSliced{idx};
pSlice = do_something(pSlice);
pSliced{idx} = pSlice;
end
p = cell2mat(pSliced)
