amplitude of oscillation

It is due to the fact that when amplitude of oscillation is small, the accelaration of the end point of the spring where we hang mass is also small, where the value is much smaller when compare with the accelaration due to gravity. But when amplitude of oscillation is large, the accelaration of the end point of the spring where we hang mass will be large, where the value is comparable large when compare with the accelaration due to gravity. Thus at some instant, the slotted weights will 'feel' reduced gravity, and as they are not adhesived to the spring and holder (i.e. they just placed on the holder), they may be shaken off. If you want them not to be shaken off, you may have to stick them to the holder by adhesive tape or glue.
I hope this can help with your understanding. =)

2007-12-11 12:00:59 補充：
The direction of acceleration is vertical, same as gravity. The weight holder is moving so quick that when it is moving downward, gravity is the only downward force act on the slotted weight, but gravity and 'downward pushing' from the spring are acting on the weight holder.

2007-12-11 12:02:35 補充：
The downward acceleration of the weight holder is to some extent bigger than the slotted weight, as a result, the slotted weights may be shaken off if they are not fixed to each other.

I understand it now too...
thanks =]