PLS-00302 component must be declared

 Hello all,
i have 10GR2/standard edition linux 32 bit. I created  a test procedure

CREATE OR REPLACE  PROCEDURE "IS"."POKUS"
IS
BEGIN
DBMS_OUTPUT.PUT_LINE('TEST');
END;
 
with userv IS working this procedure fine
execute IS.POKUS();

and  with created user FREDERICK with
grant execute on POKUS to FREDERICK;
working fine

but if i try execute this procedure with user BOB with
grant execute on POKUS to BOB;
will make error
PLS-00302: component 'POKUS' must be declared

When i try make synonym will execute fine
CREATE SYNONYM "BOB"."POKUS" FOR "IS"."POKUS";
execute BOB.POKUS;

Where is problem ?
Thanks a lot