When a ball drains the game does not notice it, stalls for a while, then finally counts the bonus. Switch test shows the number four up-kicker (VUK) opto stuck closed.

Drained ball not seen, game stalls: VUK opto stuck closed, replace the transmitter

Cause: The transmitting opto LED had failed even though it still glowed red; the receiver alone was not the problem.

Checks

• In switch test, block and unblock the opto by hand and see if the switch changes.
• Reflow the opto board header pins and make sure the wires are fully pushed into the IDC connector.
• Check the transmitter and receiver are lined up.

Fix

• Replace the transmitter LED, and ideally the receiver at the same time: current replacement optos differ slightly from the originals, so fit them as a pair.

Systems

• Sega/Stern Whitestar

Games

• The Lost World Jurassic Park

Related tips

• Alien head opto stuck on after cleaning: replace the opto pair
• Two balls launched at once: check the game has the right number of balls
• All balls are kicked into the shooter lane at power-up: trough opto board solder joints

Source: Pinball Repair help group!! (Facebook group)