This doesn't strike me as a probable answer, but we bought a Scotts rider from HD couple years ago. Used it and though it "cut" the grass, I was never very satisfied with the cut. It did an especially poor job when the grass was a bit longer, not unlike your situation.
It was a returned item to the store when we bought it and as such, we got a fantastic price on it (like $900 for what was on their floor for about $1500..so I could live with a "poorer" cut). /forums/images/graemlins/grin.gif
The next spring I went to put new blades on it (lots of rocks), and low & behold, seems the ORIGINAL blades were on backwards /forums/images/graemlins/blush.gif . it was never "cutting" the grass as much as ripping it. Now I knew why it was returned to HD. /forums/images/graemlins/grin.gif I put the new blades in the tool drawer, and turned the origional blades around and it cut my yard finer than a scalpel. /forums/images/graemlins/cool.gif
I figure the odds are not there that you have this situation, but might be worth a look see on the underside.
Richard