Sometimes, when it is time to nominate a new founding father, the dialog is too small to show the OK button so you can't click it. Sometimes, the Okay button is still partially visible, sometimes not at all. At least you can press Enter as a workaround.
My guess is that this is related to the text length and may only happen if the text of a founding father is longer than expected. Perhaps in a translation?
This happened with the following founding fathers in the selection when the German translation is used:
How to reproduce (probably):
Version: 0.13.0+dev, commit cb10784763ce2a594dc5333ba83a56e197bc4e19.
I was annoyed by the same problem. I have raised a pull request with a proposed solution https://github.com/FreeCol/freecol/pull/128
This issue has been fixed in the development version, and will be a part of FreeCol 1.2.0.