Shari,
That is discouraging news. I too have been trying to delve deeper into my Dad's line by eliminating my mother's matches. Unfortunately, Dad passed several years ago or he would have been all over this DNA testing. Fortunately he left me several years of genealogical research and documentation.
I would like to know more about Ancestry's criteria. I have used the Gedmatch tools but don't have Excel, so I can't use Jeff's tools. GEDmatch shows that my parents are not related, but some of my mother's matches have names and connections that I'm looking for on my father's side. It is a mystery!
Kathy