We had, more then one time, the same problems in our lab here.
And we found different reasons. Have you change any little
thing within your fixation procedur? Or are the animals from
which you take the brains another age compared with your
"older" once, with which you worked befor?
Another possibility is that the antibody is not ok. That also
can happen, that the producers sell antibody charges which
make a high background.
I hope this will help you and good luck!