It would be easier to answer your posts if you added more detail...
What format is the clip you're importing? Are you file/importing it or log and capture and/or transfer? Does the metadata show it was imported in the correct format..i.e, does it look right until it goes on the timeline? Are you setting the system settings before creating a project/timeline?
There is a little "anamorphic" check box that you can see in the bin, if there is no check mark, click the empty space & it should go to 16:9. You usually have to scroll right through the properties to find it. That is most likely your culprit.