Delta 12 1/2" Surface Planer Question

I own a Delta 12.5" surface planer and have recently noticed that the unit is cutting unevenly. If I were to run a piece through the right side of the cutter head, than run the piece again through the left side, more material is taken off. If I reverse the operation, run a piece through the left side of the cutter head and than again through the right side, no additional material is taken off. I thought the knives might have worn unevenly, so I rotated them and got the same results. The OM does not even mention any type of adjustment for the knives as they are indexed and should not move. The infeed and outfeed rollers appear to be level.
Any ideas what could be causing this and how it might be fixed?
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Fish wrote:

I have an older planner and what I had to do to corrcet a similar problem was to replace the bushings on both ends of the cutter drumI do not know if that is possible for your machine but this is some where to look
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Just to be sure, what's the model number? One model I looked at comes with knife setting gauges, which leads me to believe that the knives can be adjusted. Tom Fish wrote:

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The knives are fixed. The cutterhead assy is driven by a bicycle chain mechanism. Either the chain or the sprockets might have slipped and canted the head.
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