Under Email>Templates, edit the template to verify that the Data Source drop-down menu for the template matches the Data Source for the desired list.  For example, if a list based on a constituent query is not available, verify that the data source for the email template is Constituents.  Once Constituents is selected in the Data Source drop-down menu, a constituent-based list will be available for messages using the template.

Please note that if the template data source is set to None, the desired list will be available, but the merge fields will not be available when composing the message.  

If the data source for the email template does not match the desired data source for your list, please follow the Knowledgebase solution How to copy a NetCommunity email Template and choose a different data source