Did the Old Testament prophets always understand what they had written?

It depends on what you believe. I believe the Old Testament Prophets were writing about more than just social and political issues of their day. Isaiah for example writes about social issues of his day (chp 1-39) but then in the last 27 chapters are filled with consolation and hope as Isaiah reveals God's promise of future blessing through His Messiah.

In short no, I think the OT prophets didn't always understand the full meaning of their prophecies.