It is a little strange that a single field should not be displayed - just to be sure: you did create a column in WMP to show the composer? WMP does not show the column of its own ...
Mistakes like typos in a field name are one of the favourites, e.g. Germans tend to create a field TITEL instead of TITLE.
If all else fails: use one or more programs to check the integrity of you your files. Good candidates are MP3val, Mp3diags and Foobar2000 (and the utility context menu functions).
And yes: I see all the composers I entered. And as I used the "semicolon blank" as separator, WMP even shows each name separately, e.g. John Lennon; Paul McCartney has individual entries for John Lennon and Paul McCartney in the list of composers.