There are checks to make and steps to correct any problems.
Check that you have not loaded too many sheets into the tray. The maximum number of sheets depends on the paper thickness: around 50 sheets of plain, standard paper.
Check that the sheets are correctly loaded into the tray.
Check that the sheets are correctly adjusted and centered in the tray.
Check that you have extended the input and output trays correctly: the first extension for A4, the second for A3 paper.
If the paper is hanging over the back of the input tray as shown, it may fail to load.
Try to avoid leaving sheets of paper loaded in the printer for a significant length of time, because they will deform.