Try removing the slug rejector and cleaning all the coin paths.
Although this 'mech has an electronic "flight-deck" ( it uses an electronic detection circuit similar to a metal detector to determine the validity of the "coins" by metallic resonance ) it also may have a series of small photo detectors that would be used to determine which "coin" ( based on diameter) is rolling down the coin-path
(sometimes also called the coin-rail or "flight-path of the coins") if it does, ( have a set of photo-detectors) then cleaning the coin-path would help. there are three sets of these mini metal detection coils built into the fixed base & movable portion of the flight-deck , & the photo-sensors appear as small holes about the size of a hypodermic needle would make if pieced into the plastic, the receivers are on the fixed side of the flight-deck, & the infrared LEDS are in the movable portion operated by the coin-reject/return lever
Also make sure the "de-bounce" rail is intact & not cracked or chipped (this is an thin arc-shaped piece of ceramic or aluminum used to take the "bounce" out of the dropped/received coins after the enter the "slug-rejection" portion of an electronic type coinmech, so the "coins" can be accurately measured by the detection circuitry. if it's damaged, then some coins will fly by the detection coils too fast & be judged to be "slugs"
NEVER disconnect the flight-deck from the coinmechs main body with the power
ON, as you will scramble its tuning. & if you swap one flight deck for another, you must re-tune/calibrate it.