DIAGNOSIS The paper sensor isn't being triggered, or the paper isn't being picked up. This is often caused by the paper not being seated correctly against the rear guide, or the pickup roller being dusty.
FIX 1) Remove all paper from the tray. 2) Flex the stack of paper and fan it to prevent sticking. 3) Re-insert the stack firmly into the tray, making sure it's under the metal clip at the front and pushed all the way back until it stops against the rear guide. 4) Adjust the paper width guides to snugly fit the stack—they shouldn't be tight, but there should be no side-to-side wiggle. 5) With the printer on, open and close the paper tray firmly to reset the sensor.
IF THAT DIDN'T WORK The rubber pickup roller inside the right side of the paper path can get glossy and lose grip. Unplug the printer, open the rear cover, and look for a gray rubber roller. Clean it with a lint-free cloth dampened with a little water (not alcohol). Manually rotate it to clean the whole surface, let it dry, then try again.
HEADS UP Overfilling the tray past the line marked inside it can cause feed errors. Also, avoid mixing different paper types or weights in the same stack, as the thickness sensor gets confused.