Washing machine pipes

We have had to re-route our washing machine drain because of a cracked line under the slab. Our new lines have about a 6 inch fall from the washing machine drain pipe, then a pee trap. When the water pumps out of our machine the water is backing up out of the pipe. When we finished, we put a hose in the pipe and turned it on and had no problem with the water draining. What have we done wrong? Thank you for your replies.
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