Spotlight not searching Time Capsule drive, indexing enabled

When using Spotlight search, it is not finding a specific file, which I know is located on my Time Capsule drive, which, of course, is mounted and indexing thereof is enabled.
Please, see below relevant Terminal extract:

Martin@Martins-MacBook-Pro ~ % mdutil -s /                                                              
/:
    Indexing enabled. 
Martin@Martins-MacBook-Pro ~ % mdutil -s /Volumes 
/System/Volumes/Data/Volumes:
    Indexing enabled. 
Martin@Martins-MacBook-Pro ~ % mdutil -s /Volumes/Time\ Capsule 
/System/Volumes/Data/Volumes/Time Capsule:
    Indexing enabled. 
Martin@Martins-MacBook-Pro ~ % ls -l /Volumes/Time\ Capsule/Documents/Afrikaans                     
total 15296
-rwxrwxrwx@ 1 Martin  staff        0 Jan 15  2012 Icon?
drwxrwxrwx@ 1 Martin  staff      264 Jan 20  2012 Teach Yourself - Afrikaans
-rwxrwxrwx@ 1 Martin  staff  7761600 Aug  9 09:33 afrikaans woorden.txt
Martin@Martins-MacBook-Pro ~ % mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans -name afrikaans     
Martin@Martins-MacBook-Pro ~ % mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans/ -name afrikaans
Martin@Martins-MacBook-Pro ~ % sudo mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans -name afrikaans 
Martin@Martins-MacBook-Pro ~ % sudo mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans/ -name afrikaans
Martin@Martins-MacBook-Pro ~ % 

I have tried rebuilding the indexing by adding and removing the drive from the Spotlight Privacy tab.
What should I do? Please help!
Thanks

UPDATE 1 as per kodliber's suggestion:
Have you tried by manually import this file you mention using the command mdimport?
Yes I have, to no avail.

Martin@Martins-MacBook-Pro ~ % ls -l /Volumes/Time\ Capsule/Documents/Afrikaans/
total 15296
-rwxrwxrwx@ 1 Martin  staff        0 Jan 15  2012 Icon?
drwxrwxrwx@ 1 Martin  staff      264 Jan 20  2012 Teach Yourself - Afrikaans
-rwxrwxrwx@ 1 Martin  staff  7761600 Aug  9 09:33 afrikaans woorden.txt
Martin@Martins-MacBook-Pro ~ % mdimport /Volumes/Time\ Capsule/Documents/Afrikaans/afrikaans\ woorden.txt
Martin@Martins-MacBook-Pro ~ % mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans -name woorden
Martin@Martins-MacBook-Pro ~ % mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans/ -name woorden
Martin@Martins-MacBook-Pro ~ % sudo mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans -name woorden
Password:
Martin@Martins-MacBook-Pro ~ % sudo mdfind -onlyin /Volumes/Time\ Capsule/Documents/Afrikaans/ -name woorden
Martin@Martins-MacBook-Pro ~ %

UPDATE 2 as per Jahhein's question:
At the root of the volume, is there a file named .metadata_never_index?
No, there is no such file.

Martin@Martins-MacBook-Pro ~ % ls -la /Volumes/Time\ Capsule | grep .metadata_never_index
Martin@Martins-MacBook-Pro ~ %

UPDATE 3

Doing the same search using Spotlight via CMD+Spacebar works: