I gotta ask.......Is this Kawasaki carburetted or fuel injected?
And, with multiple dealer trips, what has the dealer done toward resolving this problem (what repairs or services have you been charged for?).

Are you mowing at wide open throttle?

Check to make sure the choke is adjusted properly.

How old is it? how many hours on it.

The X520 has a Kawasaki water cooled and carbureted twin and has only been offered for a couple or three years... not that old.

Some Kawasaki twins run poorly if they aren't up to working temp. Perhaps there's a cooling problem keeping the temp too low. Bad thermostat?

Like justalurker said, make sure the fuel delivery is properly adjusted and always wide open throttle, that's the way they're designed to operate most efficiently, including proper discharge of the grass out of the deck. Some people are under the mistaken idea that you can just "cruise" around the yard at partial throttle and it's easier on the engine...doesn't work that way unless you have short, dry, easy cutting types of grass. Keep the blade tip speed up to max. and stay in the upper HP/Torque curve area.

That mower has a 4year , 500hr warranty on it.

Here's something to check I found from another lawn site:

"I was mowing late this week. I mowed 4 large lawns. The grass wasn't wet. My mower would start clumping and thought the deck was full of grass or the the belt was loose. Every lawn did it even though I clean the deck. The blades were still razor sharp. I went home and I was thinking maybe the tires are low on pressure. I usually have them at 24 PSI which is the max. I checked the tires and behold they were at 21 PSI. I used my portable air machine that I leave under my seat and raised the PSI. The next day I went out and did not have a problem with clumping. Not sure how I lost that much PSI I usually check it every Month or less."

And a reply to the above was as follows (makes sense):

"Yes, your thinking is correct, tho it's not so much that it's cutting lower it's that lower pressure in the rear tires, or fronts, will put your deck pitch out of whack making the cut quality decline. Yes, it will cut lower as well, but that shouldn't effect the deck clogging up much.

"I agree 24 PSI is a LOT of pressure for mower tires. In the rears most usually run 20 PSI tops, I run 15 to 20 depending on the mower. I use a pressure gauge like used for setting pressures in racing tires. It's very accurate and reads in 1 PSI increments, plus I can bleed air out with a push button to get the pressure set. Crazy? I don't think so... I can tell if my tires are low by a few pounds due to cut quality/mowing a fuzz lower. Those tires are soft and expand/shrink a lot with a small pressure change. In the fall is when I find I have to check and adjust tire pressures a lot. Cold nights/mornings they lose a few PSI pretty fast.

"It's all in what pressure your tires were set at when your deck was leveled. Go by the factory recommendation, set all 4 tires exactly, then level the deck front to back and side to side (not "level" front to back, but with the correct pitch as in the manual)."