Hi marcd,
Please check if your theme is using wordpress function the_content() function to output the post content,
if it isn't , you can goto your WP admin side, Views-> Settings,
in section "Theme support for Content Templates", fill the function name of your custom theme.
Content Templates modify the content when called from the_content function. Some themes don't use the_content function but define their own function.
If Content Templates don't work with your theme then you can enter the name of the function your theme uses here: