It may be just as easy to just write down a filename.
In principle you can import information from a filename with the function Convert > Filename - Tag.
e.g. if you have a filename like
Beatles - Let it be
then enter the following pattern for the converter:
%artist% - %title%
I could give you a more accurate example pattern if you told the exact filename.
I cannot answer the question why some files of yours have missing information.
You can filter (F3) out the missing info by using:
title MISSING
Or
artist MISSING
Or to get both the artist and title with missing info use:
artist MISSING AND title MISSING
Then use whatever pattern suits the filename to fill in the info or you can use a script to fill in that info automatically and more. Scripts can be found here: /c/web-sources-scripts