Another thought . I don't know whether you purchased Yamaha OEM plates ,
but some of the aftermarket plates are thinner than yamaha plates .
When you stack five or six of them together , the shortfall can be 1 or 2 mm .
This effectively reduces the spring pressure available , possibly enough to allow slippage .
I seem to remember this occurring with CT3's before , but I could be mistaken .

^^ +1
Even Yamaha made the disks in different thicknesses... I've seen 2mm and 3mm thick disks.