What fixative are you using?  Formalin is not recommended for this 
antibody by PharMingen, but the zinc TRIS fixation (non formalin) is.

Some people have been doing some different fixation times for various tissues,
just to make sure ZnTris is optimal.  I have seen a few hours, to overnight and
longer, at RT.  Because some antibodies are finicky, you may even have to
do a fixation time study or even resort to frozen sections/acetone fixation.
I would also avoid processing tissues in solvents that have had formalin
fixed tissues passing through!  Think about that when working with mouse
tissues fixed with this nonformalin fixative.  

Guess that is why we stick to our frozen sections.  PharMingen tests their
antibodies acetone fixation, and probably now does the zincTris fixation
which they give in their catalog.  In general, their antibodies give 
excellent results IF you follow their recommendations, with few problems.

A PharMingen fan, 

Gayle Callis