Hide content shorcode

Hi

I am trying to create a shortcode which would hide content between the shortcodes.

For example:
function mcp_premium_content( $atts, $content = null ) {
if ( is_user_logged_in() && !is_null( $content ) && !is_feed() ){
return $content;
} else {
return ”;
}
}

And now the following can be put into any post, page, or widgetized area to filter who sees your content.

[premium]**Any Content Here Can Only Be Viewed
By Registering and Logging In Users**[/premium]

But this still seems to return all page content.
I am creating the page using
[av_textblock ]
[premium]
[/av_textblock]

[av_one_full first]
[av_textblock]
Lorum ipsim
[/av_one_full first]
[/av_textblock]

[av_textblock ]
[/premium]
[/av_textblock]

Is this because of nested shortcodes clashing do you think?
Is there an easy way round this?

thanks