Where to enable Automatic Child Variant?

[jiapei100]

Hi, all:

Joomla 2.5.7 + VirtueMart 2.0.10 .
It seems Automatic Child Variant is not defaultly/automatically enabled in VM 2.0.10.
Some of the products in the same group (with the same father product) can show up such an option;
for others, no selection at all...

Why is it so? and How to have it solved???


Cheers
Pei