It seems to me like a fixation problem. Try increasing your fixation time or changing your fixative solution.

The problem youre seeing could also depend on the thickness and permeability of your sections; you may need to permeabilize the sections a little (with a detergent or with citric acid solution) to let the antibody go into the cells, especially if you are looking to stain histones as they reside in the nucleus which is a double layered membrane within the cell. Your observed DAPI would be getting through faintly because it is only slightly membrane permeable in fixed cells. Hope that helps!

I'd try a better permeabilisation as stated above (I use triton for heart cryosections).

How thick are your sections? Do you use a sufficient amount of solution when stating your slide? Do you put some parafilm on top of it while staining to ensure homogenous repartition of the solution?

Thank you everyone for the suggestions! I have tried permeabilization with Triton X. I circle each tissue section with a oil based pen then cover them with 30uL of solution. I incubate my primary antibody O/N covered in parafilm and in a humidity chamber. My sections are 8um thick.