Basic gravity fed loaders can only keep up with about 5 bps. Without agitation the paintballs will stick together in the loader and jam up causing feeding issues. This tends to be worse while the loader is closer to being full and the increased weight causes more sticking. You will need to shake the hopper after 5-10 shots to make sure paintballs are being fed into the marker.
Using an electronic loader like the VSL or Vmax+ solves this issue by agitating the paintballs automatically after every time you shoot. This keeps paintballs always in the marker ready to shoot.