STARTER toner cartridges by Brother (the ones that come with the printer when you first buy the Brother laser printer) have "gears" installed in the cartridges, which need to be replaced when the cartridge empties.  When these starter cartridges empty, these "gears" click out of place, to signal to the printer that the toner is "out" or "empty".  So they need to be replaced, and replacement "reset gears" are available which are easy  to install.  These gear replacements are not needed for non-starter toner cartridges.