I found and tried this:
http://forums.macrumors.com/showthread.php?t=1327936
where "pmau" suggest these commands:
a) List available plugins on your system
mdimport -L
b) Show the status of the indexing process:
mdutil -s -a -v
c) force clean rebuild of the index (after you posted the output of above commands)
mdutil -a -E
d) Check /var/log/system.log
you might look into that file to check for message of spotlight, or mds the importing daemon.
And the result are these messages in the system log:
Sep 23 21:36:18 applik com.apple.mtmd[2175]: Spotlight cannot end the session: 1, for /
Sep 23 21:39:49 applik mds[2113]: (/.Spotlight-V100/Store-V2/9D4D7FF9-1CE2-44E3-A3F8-223A14835999)(Error) IndexStore in bool setLocalizedAttributesLocked(datastore_info*, db_obj**, const __CFString**, const __CFDictionary**, CFIndex, StoreOptions, const __CFDictionary*, bool, bool, const __CFArray*):Error:7 setting field:com_apple_system_prefs_keywords for oid:19091733
Sep 23 21:39:56 applik mds[2113]: (/.Spotlight-V100/Store-V2/9D4D7FF9-1CE2-44E3-A3F8-223A14835999)(Error) IndexStore in bool setLocalizedAttributesLocked(datastore_info*, db_obj**, const __CFString**, const __CFDictionary**, CFIndex, StoreOptions, const __CFDictionary*, bool, bool, const __CFArray*):Error:7 setting field:com_apple_system_prefs_keywords for oid:19106139
Sep 23 21:39:57 applik mds[2113]: (/.Spotlight-V100/Store-V2/9D4D7FF9-1CE2-44E3-A3F8-223A14835999)(Error) IndexStore in bool setLocalizedAttributesLocked(datastore_info*, db_obj**, const __CFString**, const __CFDictionary**, CFIndex, StoreOptions, const __CFDictionary*, bool, bool, const __CFArray*):Error:7 setting field:com_apple_system_prefs_keywords for oid:19109698
Sep 23 21:39:57 applik mds[2113]: (/.Spotlight-V100/Store-V2/9D4D7FF9-1CE2-44E3-A3F8-223A14835999)(Error) IndexStore in bool setLocalizedAttributesLocked(datastore_info*, db_obj**, const __CFString**, const __CFDictionary**, CFIndex, StoreOptions, const __CFDictionary*, bool, bool, const __CFArray*):Error:7 setting field:com_apple_system_prefs_keywords for oid:19103413
Sep 23 21:40:03 applik mds[2113]: (/.Spotlight-V100/Store-V2/9D4D7FF9-1CE2-44E3-A3F8-223A14835999)(Error) IndexStore in bool setLocalizedAttributesLocked(datastore_info*, db_obj**, const __CFString**, const __CFDictionary**, CFIndex, StoreOptions, const __CFDictionary*, bool, bool, const __CFArray*):Error:7 setting field:com_apple_system_prefs_keywords for oid:19120109
Any ideas, please?