为什么bootstrap-select插件不能使用ajax

I have created dropdown list for category and from that category after that I created another dropdown list for subcategory using ajax it works fine. But in second dropdown I want to select multiple option for that I am using bootstrap-select plugin. From this plugin I am using selectpicker class. On any file this plugin works fine but on ajax response page it does not show the effect of that plugin selectpicker class. Why is it so? please help...I have created dropdown list for category and f