Over-stimulation will result in a lot of cytokine/protein being secreted by the cell. This will produce spots that will start to merge and become indistinguishable. Reduce the amount of stimulant in the culture media or culture for a shorter amount of time.
Reduce primary antibody concentration.
The longer the cells are incubated, the more cytokine/protein they will secrete. This will result in larger spots that will start to merge and become indistinguishable. Reduce cell culture step incubation time. (It is generally advised not to exceed 24 hours).