Hi Bruce,
During the thirteen years that I have done immunohistochemistry
(manually), known positive control staining failure has happened maybe
10 times and it has always been due to human error, ie. omission of
primary or secondary antibody or streptavidin. It is easily done when
not paying full attention to each slide. I make the Tris buffer in 20L
quantity and it lasts approx. a month. I have also experienced
diminished staining with certain antibodies when this buffer is not made
properly, ie. pH is somewhere around 10 instead of 7.6. I hope this
gives you some direction to go, it is always best to try troubleshoot
any problem immediately while everything you did is still fresh in your
mind. Katri.

Katri Tuomala
Anatomic Pathology
St.Joseph's Hospital
Hamilton, Ontario, Canada