A cause of non-staining in the outer layers of a sample may be delayed fixation and therfore drying and destroying the antigen. A version of this is the non-staining in specimens of electro-cautery.

Another thought. Are the reagenses spread regularly on the whole section, or is there some kind of drop-forming?

I have used a grease pen so there is a droplet, but I do leave about ~1cm between tissue edge and grease pen mark so the edge of the sample is not right at the edge of the droplet. I have also observed this effect in tissue microarray format in spots that are located ont he interior, which to me argue against some type of "droplet edge" effect. 

Matthew where do you get your FFPE tissues from?

Could be a problem with how the tissues were processed, emersion fixation either too long a duration or too high of a concentration of fixative.  Both could result in the exteriors of the tissues having lack of staining (ie the antigen has been destroyed) and those in the center are more “protected” and therefore more viable for antibody – antigen interaction, thus DAB positivity.

I have done a tonne of DAB staining myself but have never encountered this problem. We use this hydrophobic pen (https://www.vectorlabs.com/catalog.aspx?prodID=15) to make sure that the section is completely covered in the reagent. We use this kit (http://www.dako.com/us/ar49/p234877/prod_products.htm) for the DAB staining. Though it is expensive, it gives best results and contrast every single time! Good luck!

How are the tissue array bits obtained? In comparison to fine needle biopsies, there are also found edge effects sometimes. In that case the "high speed needle" is said to produce too much heat because of high compression at the edges, which leads to decreased or no antibody staining.

I really don't know, if this is applicable on your problem. But seeing this clear cut on your picture, I assume, that something happens to the antigen at the edges.

Thanks for all the suggestions.  Am trying a repeat today with side-by-side sequential sections on each slide ....should at least confirm if is coming from specimen or staining procedure.

Matthew, could specify if your sections are floating, cryosections or paraffin-sections. Some times we have this problem with cryocut sections (due to cryo conservation or freezing sampl procedures).

Do you encounter this problem in all your slides or only those made from a particular block? If you have this problem in all slides in the whole IHC run (made from different blocks) - then it is a problem of non-covering by the reagents (on any step) of the whole cut. If you have it only in slides, made from one particular block - then it is a problem with tissue processing/embedding. 

P.S. And I hope you use a humidity chamber for incubation?