It's hard to say what could be the issue without knowing a little more.
First, do you have a feature image set for the portfolio items you're trying to display? FPW will only try to show the feature image, not other images attached to the post.
Second, are you using the default widget template that is included with the plugin or are you using a custom template? If you're using a custom template, make sure that the
post_thumbail code is properly placed as it is in the default template.
Third, have you double-checked the HTML source of the page where the image should appear? The image could be in the source, but your theme's styles may prevent it from appearing.
Hopefully one or more of these ideas will get it fixed for you. Cheers!