Missing cells from immunopanning
Immunopanning in my experience takes an awful lot of optimization, but is worth it when it works.  I would plan, if you have the resources, to sac rats just for assay optimization before you even begin to try to get real data out (assuming your planning for a steady stream of primary cells).

That said, there are some real easy checks you can try do first.  Try to get a rough cell count on what you put on your negative plate (depending on your prep steps, that can actually be a bit tricky), and then count the cells again as you transfer the cell suspension to the positive plates.  If you don’t have any cells left, or you have very little cells left, then you might be losing all your population on your negative plate.

If you do have cells in suspension, I would start to question your positive plate.  Are you making them yourself or are you buying them coated? Which rat strain are you using?  Perhaps you need to titer your antibody concentration on the plate.  There are really a whole lot of steps in immunopanning that can be optimized.