How do I adjust for even cutting?

Possibly blades need to be sharpened.also,you may be trveling too fast.only the first inch or two of the blades do the cutting.if the grass is higher than average,and you travel too fast,and the blades are dull,the unit cannot keep up with the rat of grass it has to cut every millisecond.hope this is useful.if it persistz,try being more specific and i will try again.i am a certified small engine mech.thanks,bart

yes that is most likely the cause of it, also, you have to cut more often when mulching, and the higher the cutting deck, the more likely the grass will blow out without getting mulched enough. More than likely, the blades are dull, and/or the grass is too high, or possibly even wet if it is high.

Maybe the grass you are trying to MUCLH is a little too high. Mulching doesn;t really cause the machine to "shoot" out the grass the way conventional cutting with a conventional blade does....it is the type of blade that when spinning, creates air pockets under the deck that allow the blades to keep cutting the grass over and over. Switch to a different blade to get it to shoot out.

the only way to fix it is fit the right blades
they have fitted the wrong blade's to it,,,
the blades should overlap when cutting the grass they cannot crash into each other as there are (timed up) not to do that the new blades they fitted are to short so you have no over lap on the gutters that leaves a strip of grass left uncut between the blades,,
call them out to put this stupid mistake right,,,

Your forward speed may be too fast for the mower blades to cut every blade of grass. It may be new, but check the deck to see if there are any dents pushing the quill assemblies up making the deck cut uneven. Or it may have the wrong blades on it.
I have seen this problem in the past with a Gravely mower. The person was trying to cut grass to fast. He sold the unit and the new onwer did not noticed any the problems the old owner described.

If the grass is very tall (more than 10") you will have to slow down to less than 1 mph, if you have a mulch kit on this unit, remove it to get the grass out as quickly as possible. Also, sharp blades & a level deck are important.

change the blade

check the condition of your righthand spindle, if spindle has no play, your deck may need adjustment

It sounds to me like a very loose belt or very worn out belt and also an adjustment problem.