Even though your system may show that your preferred tray (usually 1 or 2) is set to 8.5" x 11", occasionally the printer will start looking to other trays to find that size. The fix for this is usually very simple.
1. Open the tray
2. Make sure the paper is in a tight stack and not staggered along the edges.
3. Adjust the tabs that hold the paper in the tray so they are tight against the paper. If the tab is off by even 1 notch, the printer may sense that it is not set to 8.5 x 11 and look elsewhere.
If the above steps do not resolve the problem, please submit a help desk ticket.