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That is most likely to be due either to broken or aging parts in the back, or 'duplex', are of this printer - or perhaps to aging/slippery roller surfaces in that area.
To try to treat roller problems, first, inspect the rollers etc. in the back area of the printer, for any that won't turn easily at a touch (except perhaps for the ones mounted on a solid metal shaft, that only the printer can turn, when it is in motion), or rollers that have come loose, or part that are loose or obstructing the feed path, etc.
Then, try a roller cleaning, after removing the toner cartridge/drum from the printer and setting it aside in a dust-free dark or dimly lit place, e.g. on a clean piece of paper in an empty desk drawer:
If the problem might possibly be owing to aging roller surfaces (for paper is fed into faxes and printers with rubber or plastic cylindrical rollers, that propel the sheet into or through the machine)... then you might need to clean the rollers in the problem area where feeding is not happening.
Remove all paper from the machine, unplug it, and open it so as to have access to the rollers in the problem area where feeding is not being done properly.
Take a clean high-quality lint-free rag cloth, moisten it with rubbing alcohol, and rub the rollers firmly all over their paper-contacting surfaces, three times over.
(If the rollers are hard to reach, there is another way to clean them... let me know if that is the case, and I will detail how to do the cleaning.)
Allow 5+ minutes for drying-off, then reload the paper, close up the machine where you opened it up, and then try again.