i had a 2-speed Kreidler for a while. There were two centrifugal clutches, one right behind the other. One is the main clutch and the other is the shifting clutch. At a certain RPM, the second clutch would grab and engage 2nd gear.
If all the clutch pads are OK as far as thickness, and clutch springs are ok, i guess, there could be an oil leak from a seal or something and oil has gotten on the #2 clutch pads.
It sure seems like you have a slipping clutch.