just wondering if the blade might be on backwards..the blade also has air fins (the bend up)that help pull up the grass to recut it before dropping on the ground.
i've got the last years model of a 6.5hp - 3 in 1 cutting deck(mulch,side,bag.)and set throttle.self propelled walk behind.
and i usually let the grass get long(like 3-6 inches from the ground). and don't have any problems using the mulcher on this tpye of grass. i also don't make the grass real short at the cut.