Multiple selection from a CheckBoxList - how to insert mutiple rows ?

Hi there
I haveCheckBoxList called (ID="CheckBoxList_sub_dpt") This component bound to a SqlDataSource, so populated dynamically. The user can do more than one selection (ence the use of a CheckBoxList)
The concept is like this:  A user select a Category from a DropDownList when this is done some sub-categories appear in the form of my CheckBoxList.
The user should then select one or more sub-categories. When the form is submitted I should have one row per sub-category selected  [ the row would tie the user_ID and the Sub_Category_ID ]

 The problem I have is that when a user select more than one checkbox, it only insert one row in my table ( it will insert the first checkbox selected)


 Here is my bit of code (only the relevant bits):
ON THE APX PAGE:

 <!--DS (SqlDS_dept) populates the drop down menu "choose department"-->

<asp:SqlDataSource ID= ...