Unfortunately, your Manage my Home profile was not completed so I am not sure what mower or bagger you have and cannot give you any exact reason why only one bag would fill. But, there are a few general things that cause issues with many baggers.
First, it is important that you cut the grass frequently and cut when the grass is dry. If not, the cuttings will be too large or heavy to fill the bagger properly.
Other common issues are chutes that are misaligned or bins installed in the incorrect positions. Also, if the bagger assembly itself is not mounted correctly the angle could be wrong and grass may fill one bin faster than gravity can drop grass to another. And, on some mower decks it is critical to have the correct baffling and blades installed or grass will not lift to the bagger with enough force to fill properly.
Normally, the owners manuals for your mower and bagger will have clear instructions on how to install the bagger for best results. Be sure to update your Manage my Home profile and you can view product manuals online for further help.