Unable to view snapshot on error in Controller and VuGen

Loadrunner version: 8.1.4
OS:Windows 2003 enterprise and XP

Runtime Setting ->Miscellaneous ->'Generate snapshot on error' is checked;

After replay in VuGen, try to click‘View replay snapshot only’button on the scritp which the error occured,but the button is disabled;

After running in controller, double-clicking on the line at which the error occurred, there is no snapshot open, and the 'snapshot' button is disabled.

I have no idea why? BTW, Firefox, Netscape and IE 6 are all installed in the computer.

It's too difficult to analysis when error occuring without 'snapshot on error', pls help me.
Thanks in advance.