Worse, when I checked my 52 4th cousins, most of whom do not have trees, I found two cousins with trees containing ancestors that exactly match to ancestors in my tree. So I have no idea why Ancestry did not identify these as Shared Ancestors.
I have a situation like this where a 4C1R does not show up as a Shared Ancestor hint despite having the right common ancestors in his tree. In that case the reason is he has an error further down his tree. He has two wives for his great-great-grandfather that are actually the same woman - he has his great-great-grandmother twice, once under her maiden name and once under her married name, with a 5 year birth year difference (possibly because she was fudging a bit on the census). He has attached himself to this fictitious "2nd" wife, so it does not appear from his tree that he descends from the common ancestors even though they are in his tree.
That's a fairly blatant example, but all it takes is a spelling or date difference somewhere in the chain and it can prevent a Shared Ancestor hint from occurring.
