I've mounted a samba share and set the access flags in the fstab using "file_mode=0664,dir_mode=0775".
But suddenly, I've encountered a folder where most files had 644 (as defined) and some were set to 444!
Even worse:
I couldn't simply "chmod 644" on those files, because I was greeted with a "Permission denied" error.
[SOLUTION]
"444" = "r--r--r" and means "readonly".
So, when checking the access rights on the Windows side, the "read-only" flag was checked. Unchecking it solved the problem.