There was a tech note about this, mainly to do with the spring tension arm that you remove when you put a new belt on. The two metal dowels must be hard up against the adjacent plate. Also use a ruler or something to make sure the sides of the belt unit are straight.
Also, there is a tech bulletin showing how to adjust the roller for walking. If you did not have a problem before replacing the first time, then I agree with Random.
If it was the original belt then might want to look at the bulletin.
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