Had this same problem, found a solution mentioned on Verizon's forums that someone found on the MOTO Forums.
Post #6.
Phone will not pull down OTA Update - Verizon Wireless Community
"
Menu>Settings>Data Manager>Data delivery>Social applications
Make sure that the
Sync over Wi-Fi only setting
is NOT checked. If it is than uncheck it and try System updates again."
Adding to this: When you un-check this, reboot, and make sure you aren't coonnected to a USB charger (both of which prevented mine from updating). I was connected to charge via USB, and my SD card was unavailable, which may have also prevented the download. When I did all of these things, the download commenced.