What are the sources about Agnes Brock? I just found a source (http://books.google.com/books?id=zPnD0kvrLf8C&lpg=PA397&dq=...) saying John & Agnes Wiseman married about 1521, and since Agnes Brock was born a decade earlier, it seems unlikely she was their daughter. It also lists their 10 children, none of them being named Agnes. On top of that, Agnes Wiseman (Josselyn) would've been about 11 years old at the time of giving birth, making it seem even less likely.
I find no reliable sources online documenting that Agnes Brock was the daughter of John Wiseman and his wife Agnes (Josselin) Wiseman. As you correctly point out, Ernest Flagg's *Genealogical Notes on the Founding of New England* lists ten children--none of them an "Agnes":
So unless Flagg somehow overlooked her name, unless one of the six Wiseman daughters can be shown to have gone by the name "Agnes," or some other defensible explanation comes to light, the "Agnes Wiseman" in question was not the daughter of John and Agnes Wiseman.
However, we seem to be on fairly solid ground that an Agnes Wiseman was the wife of John Brock (com. Essex) and the mother of William Brock of Colchester, among other children (p. 554):
John Brock and Agnes Wiseman also appear in the following publications (snippet views only available online):
If I've figured correctly, John and Agnes (Wiseman) Brock are my thirteenth great grandparents, but I can't go back any further for either of them. :-)
Joseph Dillon Ford