Why don't prophecies always come true?

Old Testament prophecy is more about God's plan than about the fulfillment (of the details) of that plan. Sometimes the plan is fulfilled - eg Cyrus - and sometimes it's not - eg the destruction of Ninevah. As I see it, sometimes God lets people in on his thinking, but without revealing the factors that he knows will arise and cause him to change the outworking of his plan.

So the most helpful way to look at Old Testament prophecy is to consider the message of the prophecy rather than its fulfillment. I would argue though that sometimes the message is necessarily bound up in its fulfillment - eg messianic prophecies - but I guess considering the message is the best first step.

This is mainly from a first year Old Testament lecture by Jansen Condren, but with a bit of embellishment from me. I like it, but I worry that it's a bit too neat - maybe it provides a logical resolution where the biblical facts should simply be stated and allowed to sit as a paradox. I dunno, I just feel like there's been times when I've thought that this explanation hasn't been borne out biblically . . . Any comments?