After the drop target bank resets, the fourth target falls back down on its own.

One drop target drops by itself right after the bank resets: fit longer return springs

Cause: The target's return spring was too short to hold it up against the reset bump.

Checks

• Compare the springs on each target in the bank; a shorter or stretched one stands out.
• Check the target moves freely and the reset bar isn't bent.

Fix

• Replace the return springs with the correct, longer ones so each target is held up firmly.

Systems

• Bally MPU AS-2518-17

Games

• Mata Hari

Related tips

• Drop targets will not reset or stay up
• MPU stops after 7 flashes unless the top saucer coil is unplugged: bad driver chip

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