This image shows that you need to hit the Vari Target all the way back to light the special:
https://www.ipdb.org/showpic.pl?id=1290&picno=31127&zoom=1

The schematic confirms that. The 6th Vari Target position should activate the DD/Special Target Score Control relay which turns on the special light. It should stay lit until the ball drains.

/Mark

I don't know how Jet Spin tracks the bonus. Can it show a bonus of 11,000 for example? The playfield has a row of bonus value lights from 1,000-10,000, but the schematic shows the Bonus Unit has 15 positions.
Jet Spin Roto Target Special light (resized).jpg
In any event, the Roto Target Special light should come on when the Bonus Unit is in the 15th position, or 5 steps after the 10,000 bonus value light comes on. If it doesn't, check the bulb. Or, jumper around the bonus unit by clipping an alligator clip test lead from the maroon-white wire to the brown-white wire to see if the bulb comes on.