@korafola
Check the database to see if the songs are marked "unavailable", which means the file can't be read.
https://wiki.strawberrymusicplayer.org/wiki/Accessing_the_database
sqlite3 ~/.local/share/strawberry/strawberry/strawberry.db
select url, unavailable from songs where unavailable = 1;
if the songs appear there, then check if the songs are valid with an utility:
https://wiki.strawberrymusicplayer.org/wiki/Validate_audio_files
If there are no issues with files, then check the files with the tagreader utility witch comes with taglib.
If the files are read with tagreader. A full collection rescan should solve it if the files are valid and can be read.
Start strawberry from the a terminal window, do a full rescan and check for errors.
If you are using taglib older than 1.12 you should upgrade taglib. Some distros still use TagLib 1.11.1 which is an 7 years old release, so file a bug report to your distro in that case.
