Or encountered an issue like this involving a similar line?

I have performed multiple Fos-immunohistochemistry experiments in this mouse line using different Fos primaries [1:1000], trying different incubation times, switching secondaries (488 and 647), imaging on a confocal at different exposures etc.

Despite seeing clear Fos-expression elsewhere in the brain, I am unable to visualise it in regions where the CRH cells are densely expressed. In comparison, wild type (b6) mice that were run at the same time and under the same conditions showed very high Fos-expression in these regions.

Does anyone please have any advice or suggestions as to what could be going wrong?