how many spacers have you got in that gear box? as you've charged the reduction you would need 2 of the metal ones rather than one. as originally they had 2 spacers when they had the plastic first stage reduction, and then if you changed it to metal gears, you probably had to remove them. But when you changed it to 24:1 reduction, the gears are thinner so you need more spacers. The spacers should give the pins less room to fall out.