The roaming extra ball target is triggered when all qualifier shots (ELF, DWARF, MEN, Radagast, Bilbo, Gandalf) are made in a row, "before" starting a mode.
Starting a mode before hitting all qualifiers will reset the requiring qualifiers for the specific mode and you will have to collect them again.
Go to settings and somewhere there is the difficulty adjustment for modes. If standard is activated (what mostly is), at the start of a game the machine triggers a random qualifier, so that you can start a mode directly from the beginning. This already activated qualifier will NOT count for the roaming extra ball target, which means you have to start a mode first to nullify the already given qualifier.
Either this method, or It'll only be triggered when no qualifier is lit and then you hit all six.
When you change the difficulty settings one step harder, no qualifier is given at the start of a game and the extra ball target should immediately be triggered when the last qualifier is shot.
Feel free to correct me if I'm wrong, but this is how it works on my machine