For uploading several files to the same project, which is quite possible, it is important that the files have exactly the same columns and column headers. You can see the current list of columns and headers for a project on the 'edit' project screen which you can reach by clicking the 'edit' button of a project on the main projects page. 

To add more data to an existing project, go to the edit page of the project and then add the new file. Make sure columns and headers are correct.