Digital certificate not loading on machine

[Oberoi]

HI
I am running java applet on client side. The problem is that it is running on some machine but on other machines the digital certificate does not popup but the applet buttons appear. The moment i click on any button the applet area becomes black square. The jre version of that machine is 6. Any idea why the this is happening ?
Does client required any ports to be opened for viewing/accepting the digital signature certificate?

Any help would be appreciated
Thanks

[Eranga]

What your applet really does? Accessing certificates attached with the machine?

[Oberoi]

The applet is a voice recorder so in order to access the client resources, digital certificate asking client permission is required

[Eranga]

Did you check security in Java Console? There are settings like online certification validations and so on.

[Oberoi]

I enable the verify online certificates and grant permission to content from untrusted authority as well but no luck.