I've had luck in following up with some vendors when miles haven't posted, but you might want to be prepared that it could take a couple of months for them to post. Don't know why, but sometimes this can be a really slow process.

If you really don't want to loose the miles, and you have allot of them, as a last resort you can always go to United's web site and purchase 1000 miles and you'll be good for another 18 months or so.