Need help with trailer sway.

I have a 4×6 tilt trailer that I used to haul a Polaris 330 trail boss on and it pulled fine, but when I load my 2016 Grizzly on it, I will get a lot of trailer sway. The tires are properly inflated, the trailer rides level from tow vehicle. I load the Grizzly on forward, what should I check? Thanks!