not exactly sure how the jobo tanks work but every tank ive seen you need to fill it to capacity to evenly deelop the film, regardless of the amount of film. If you only have 1 or 2 sheets to develop maybe try tray processing.

The point of the tank being full is that the 1 or 2 sheets you have in the tank need to be covered with developer at all times or else youll get very uneven development.