Why LonglistSelector not load all item?

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Windows-Phone-8

I’m in ridiculous problem with app for windows phone. My app contain two page one for category (master) page, it display a list (LonglistSelector) of category. User navigate to detail page by tapping on an Item/category. This page display list (LonglistSelector) of image as gridview with respect to category.
LonglistSelector in detail page can display all image in list. But if I navigate to master page again and select a category that is already displayed before in details page, only 20 number of image is loaded rest of display loading indicator forever. If you have any idea about my mistake/solution please don’t hesitate, I've been waiting......