Chech and see if paper is actually not sticking to drum on side one (paper should stay flat on transport), then watch side two if it lifts try replacing static charge unit (if not already done)or HVT
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Copiers are only as good as there last service
The paper quality maybe the problem of the jam, try to change another brand of paper or heavier paper(80g). If paper jam still occur then try to use A4R paper to perform double side printing.