Your garbage is too light.
If you're only filling half a can, put everything in a single large bag, and put the heavy stuff on top. The heavy stuff will start to fall when they dump it, dragging the bag with it, and the light stuff will follow since it's all in the same bag.
If that doesn't work, make a spacer out of scrap cardboard that wedges in the can and takes up half the can, putting your garbage at the top of the can. The garbage will dump but the spacer won't because it's light and wedged in. If you have more than half a can, take the spacer out.
If you don't want to do that, put all your garbage in a plastic bag, and run the garden hose in the bag. It will make your garbage heavy so it comes out of the can when they dump it.