Dozens of times! You need more weight, it could be air trapped in the trailer but it's likely the bouancy of the tires.
Small lead plates screwed to the bottom of the bunks is the cleanest look but the state would probably revoke your license for life at the thought alone
It's how we solved it in Florida, remelted tire weights into 2"x12" strips and mounted them to the underside of the wooden bunks
On the uglier and cheaper side, attach a 3' section of capped pvc (won't rust) pipe that's filled with concrete to the rear cross member