When you open Product Bundles editor the software check if products info is in cache. Products info is required in order to create line-item and notification rules and actions. To save loading time the data is stored in browser cache. You can refresh the data in cache by clicking the "Products" button on the top of the editor. 

Anyway, if data is not found in cache it will be loaded. This can take few seconds, depending on the amount of products in your store.