Hi Mary,
there might be an issue with the clamping force the rolling pads have against the tray - two mechanical parts contribute to grip and pull the papers through:
1. the axis around which the rolling pads rotate
- rods/shaft, which constitute the central axis of the rolling components/pads, need to be firmly joined to the rolling components firmly. a loose coupling will result in the loss of rotational control - imagine you're trying to turn the door knob without a firm grip.
- take a look at the rubber-like grommet which acts to fill the gap between the joining of the rod and the rolling pads. if anything is loose, push the grommet back in so that the rolling pads are fixed in position along the length of the rods.
2. clamping joint of the axial rods (mentioned in 1)
- as you can imagine, printers and similar builds require flexibility when handling something fragile. this is achieved by multiple joints which allow slight degrees of variation in whatever they are clamping. so in this case, if you used A4 paper in one print job and then a thicker photo paper in another, the rod needs to have enough degree of freedom in its clamping force to allow both types of thickness.
- continuing from the example, if you have too much degree of freedom so that the clamping is essentially loose, there will be no gripping and loss of ability to pull the paper. like a clipboard that doesn't clip. take a look at the axial joints located on the side edges of the printer, specifically where the rods are being held by a more structurally fixed component within the machine, and look for any defunct components. this could include out of place springs, loose screw, etc.
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this being said, the rolling pads and the proximal feeding mechanism components may not be the issue. the the rolling pads roll when you (gently) try to rolling it with your hands (along the intended direction)? if you notice particularly strong resistance against this movement, e.g. the printer enclosure shifts or w/e, then the parallelism between the axial rods and the tray surface could be misaligned.
let me know how you tackle and if you need more help maybe post some pictures of the inside + other relevant views of the printer and its malfunctioning nature
good luck!
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