Cant install plugins and themes

Hi Guys,

Just wondering how I can get around a problem.

I installed Wordpress MU a few times already .. never any problems.. without buddypress it works fine using subdomains.

Now, when I try to install themes or plugins using the wordpress uploader or via FTP ( using correct paths as indicated ) they never install.

Eg .. with themes, I get:
eg: Theme name: andreas Stylesheet is missing.

With plugins, It just wont install .. but any other wordpress plugin works as expected.

How can we troubleshoot this problem?

Any assistance would be great.

Thanks
David