In order to use CLOB datatype with CONTAINS it should be indexed as CTXSYS.CONTEXT.
Is that trye that I need to drop and recreate this index each time after new value is inserted.
I found that if index has been created as a part of table definition and insert data later "CONTAINS" doesn't work.
I tested this as
1. drop/create table with CTXSYS.CONTEXT index for CLOB datatype
2. inserted data into the table using stored procedure
3. use CONTAINS, but no row returned
After I drop and re-create index "CONTAINS" returns data