Hi Charlie,
This is expected behavior. In absence of an IMAGETYP FITS header keyword (as happens with all DSLR camera images), the BatchPreprocessing script looks in file names to determine frame types. If a file name has any of the 'bias', 'dark', 'flat' or 'light' substrings, then it is used to decide the frame type. Otherwise there is no way to know it.
In your case it seems that the application you have used to acquire the images has not added a valid IMAGETYP keyword. You have to click the "Add Lights" button to manually add the required frames.
To check the keywords in one of the images, select it and File > FITS Header from the main menu.
