This problem can occur if one or more settings in the admin panel of PAPs have not been set for the particular product. Select the product in the PAPs admin page and make sure that all fields relevant to the PAPs display - especially the width of thumbnails and the selected image - have been set.
If you continue to experience problems, I'll be happy to take a look at your installlation if you send me your FTP and site details via the 'Contect Us' page, or by email to the address at the foot of the User Guide.