I have a problem that I have noticed in cutting this year. When I am cutting longer grass (6-7") I always have an area directly in the middle of the cutting deck that does not cut. I have a a super Z with a 60" deck. I changed to the mulch blades this year with the mulch kit and the blow out kit. I didn't have this problem last year with the standard blades. I am open to suggestions on what could be the problem. Thanks in advance.