- Have you remembered to specify that to put in the cart? The setting defaults to nothing.
- Do you have a track stop at the end with a designated dumping? Track stops default to not having a dump direction.
- What controls when to leave the loading stations? Are you sure the cart isn't just pushed to the stop and then onwards?
I take it you want a resource to production and results back loop.
- At the resource end, make sure the track stop item settings cover what you want to load. I usually leave the cart description broad and control items by having a restricted stockpile setting. Also make sure one or more stockpiles are linked to the stop.
- Unless you want to manually control it, you need to set up one or more departure conditions (I usually go for full of desired items, but if you have a complex route that's intended to pick up stuff at several places you may want increasing percentages). Also, if you have several stops, you may want timed departures as well, to ensure delivery doesn't get stalled by the station that's out of stuff ('cause the dorf responsible for production is on a break).
- At the end, build a track stop and set up a dump direction. I don't think braking comes into effect when pushing mine carts (I usually use impulse ramps along completely closed off [pressure plate controlled doors] tracks).
- At the return path, basically do the same, but with a different set of items.