I've had the same issue a few times. Even worse, on one or two occasions, I've been comming to a stop for a stop light, and can't downshift at all! Granted, I shouldn't make a habit of holding the clutch in, coasting for a couple of hundred feet, and going from 5th or 6th gear all the way to 1st in one go.
When I have had trouble shifting to first, I've had good luck just rocking the bike forward and shifting at the same time. Don't ask me why it works, but it does seem to do the trick.
"I've always been crazy, but it's kept me from going insane." Waylon Jennings