The other day, I had to rip a nine foot piece of maple, and then rout a profile on it. Since my shop is small, this meant moving a bunch of stuff out of the way, so I could turn the table saw. Bandsaw, jointer, planer, assembly cart, everything had to move, including the lumber cart. But the lumber cart didn't want to move. Had to unload the whole thing, and see what the problem was.
Here's the cart, empty, and on it's side:
Here's what happened: the cart was overloaded, the caster was undersized, and it failed:
While I had it empty, I reinforced the bottom with some plywood (hoping to cut down on the racking). Then I added substantially bigger casters...
It's still not easy to move, but at least it *does* move now...