If none of this pans out the problem... force the blades around to where they meet and see if they match up. I quit buying the eXmark blades mulching blades because the steel is too thin and weak. The blades flex under load and cause this off and on. But eventually they remain flexed down and you can see where they don't match up.
If the blades are the problem let me know and I'll tell you what to get.
If this still is not the problem, the mulch kit may have to come off because a warped deck is suspect. Some warp during manufacturing so you will have to check this with a 4' level.
But if the blades match up you can tell a lot by adjusting the deck to dead level, slipping all 3 blades to sit front to back like so ( l l l ) and taking measurements at all 3 tips front and back. Past that I can advise on how to check it with a level....
"If you place a low value upon yourself, rest assured..... the world will not raise your price."
"The bitterness of poor quality lingers on long after the sweetness of cheap price is forgotten"