You can submit your request online, as well... log in to united.com, go to your Mileage Summary, and click on "Missing Mileage Request" on the left. You can then fill out all the info for your missing flight. You will receive an email confirmation as a paper trail. I don't know if that will "spur then on to action" any more than what you've done, but since it's on the website, you don't have to worry so much about someone "losing" your paperwork...