My store categories is only allowing one choice

Hello there.  When adding a new post or editing a different post, my store categories is only allowing one choice whereas before, I could put a store item in multiple categories.  Can any one offer assistance on getting the multiple choice option as well as the little green folder that allowed me to make my own categories?

It maybe there is some JS error on the page which affects other JS functionality.

Try to check your browser's error console when it happens, it should shed more light on the cause of the problem.