What I did and which I suggest you try, is to work with different dilutions of your cell suspension. You could reduce the dilution factor of your cell suspension with the Agarose before coating on the slide to obtain more cells.
It might also be that the amount of your tissue you're using is not enough or you're losing cells during the preparation of your tissue for comet assay.
after cutting tissues(completely) by scissors, Solution produced in microtube was released for ten minute, then 10 microliters from above cell suspension is removed and mixed by LMPA and loaded on slides.