Post in lightbox plugin

Heey!

Im using a plugin to display post conte`nt in a lightbox whenever you click on a featured image. this all works fine if I just add normal text etc. But when I try to add a shortcode it just completly ignores it.

When I go to the plain page it just displays the plugin I added with shortcode. but when I open the page in the lightbox it doesnt display it.

I use the following code to load and display the content in the lightbox:

if($_REQUEST['popup']!=''){

	$postObj = get_post( $_REQUEST['pid'] );

	echo '<div class="ostContent">'.$postObj->post_content.'</div>';

Why does this code display the content I wrote but not the plugins I try to load using shortcode?

Greetings,

Merijn