I had the same problem as this post but there was no fix, so I'm posting my solution here.
When I tried to print photos on 4x6 photo paper, the printer would take a sheet, I could hear it printing, but then the paper would come out blank with ink smudges on the back. With an electric torch I could see that the rollers at the back of the printer didn't have enough grip on the paper.
It turned out that even though the paper tray was pushed all the way in horizontally, the tray still wasn't seated properly; it goes in and up slightly. Try looking at the bottom of the printer and feel if the paper tray can go in anymore. Doing this gave the rollers enough grip and I could then print successfully.
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