Snapshot problem with fast refresh

I created a read-only simple snapshot and complete refresh it OK manually with
dbms_snapshot.refresh ('snapshot','C').

I created a snapshot log in the same database as the master table, and tried to fast refresh the snapshot using dbms_snapshot.refresh ('snapshot','F'), but I got an ORA-12004: REFRESH FAST cannot be used.

Some more information:

-DBA_SNAPSHOTS

  can_use_log = YES (OK - it's a simple snapshot)   latest_refresh = 01-JAN-50 (WHY??? - I complete refreshed OK)   error = #######

-DBA_SNAPSHOT_LOGS

  current_snapshots = (null) (WHY?? - exist a snapshot referencing it)

-When I modify the master table the snapshot log records
 the changes OK, but the SNAPTIME$$ is null.
-The database link used in de snapshot's subquery has user
 system/manager, who can access master_table and  MLOG$_master_table (snapshot log)

Anybody knows what's happening ???
Thank you in advance Received on Mon Sep 30 1996 - 00:00:00 CEST