This is one of the most commonly asked questions and there are a couple of reasons why it might be happening. The culprit is usually an unticked box, or a zero valued field or a menu item not selected. Follow this checklist and see how you go:

1. Your Gallery may not have a Feature Set or Pricing Model associated to it. In the Navigation click Photos and then click the Edit link in the problematic Gallery. At the bottom of the pop-up, ensure that there selected menu items for Feature Set and Pricing Model. Press save and then refresh the page on your website to see if that has fixed the problem.
2. Have you accidentally turned on "Restrict paper sizes based on image resolution"?. This option is designed to stop customers from buying print sizes that are larger than the original photo. This is particularly useful if you upload the original resolution photos, but if you've only uploaded lower resolution photos, it will stop larger prints from appearing in your customers shopping cart options. To check, click on Website, then Settings and make sure the item "Restrict paper sizes based on image resolution" is not ticked. If you have uploaded the original resolution photos, and you do want to use this function, remember to turn it back on. Please note that "Restrict paper size based on image resolution" setting applies to Self Fulfillment products only
3. Are the Paper Sizes and Paper Types matched up in your Feature Sets? A feature set is a collection of print products that you want to make available for sale. They consist of Paper Size items and Paper Type items. Items need to be in the Active column in order to appear in the Final Product Combinations panel at the bottom of the page. If there are no options ticked in this last panel, this is likely to be your problem. Ensure that you have set Paper Sizes and ticked boxes for the Paper Type combination wand to sell. If in doubt, click the all column, press save, and see if that has made a difference to your website cart.
4. Check that your Retail Pricing has been marked up from your wholesale rates. Pricing Model's are used to set the retail prices. Check to make sure that there are no empty fields in any of the Pricing Models you have setup. Empty fields mean that the item will not appear as an option in the shopping cart. Click on Business, then Pricing Models and then click on the name of one of your pricing models. Make sure there are no empty fields in the Sale Price column.
This happens most often when you have created new wholesale items in Paper Size and Paper Type and have then forgotten to update the Pricing Model and Feature Set.