What proof do you have that Agnes was born in 1596? Have you found a baptism record from a parish record - not an online tree, hint or suggestion from the internet. I have found two in Plymouth, both at St. Andrew:
17 May 1623, Agnes, daughter of William
25 Oct 1616, Agnes daughter of John
It would be impossible for a woman in her 40s to have had her first child and survived to have more at that date.
Also what are your sources for Nicholas? I hope not an online tree.
Nell