Your FT is working correctly.
There are 3 ways to get multiball going..
shoot the Caster's club 3 times
or get the random mb reward in the Fish finder
or finish all the "boat awards" <<< this is where you are confused a
little.
The boat awards build up by making Long Casts (right orbit when
lit).
When the award is flashing, make the captive ball shot to collect.
Once the last award is collected (multiball), you can repeatedly hit
the captive ball for a point bonus that builds, and eventually light
Special.
The boat awards will not stop flashing until you lose your ball.