are u trying to create a dblink to the same database
   but pointing a different schema???
   in that case, add your global_name too.
   soemthing like,

   create dabatase link mydb.mydomain.com 
   connectscott   identified by tiger
   using 'mydb.mydomain.com' ;

   else
   create another connectstring(to the same db) and
   try the same