Index are not used

We have a table that can be queried by distinct ways according to input parameters pass to the query.

We don't want to use dynamic SQL.

These are the details:

DB version : 19.20 (ODA)

CREATE TABLE GATE_USER_APPLICATION_JOB2

  (   "ORAUSER" VARCHAR2(15 BYTE) NOT NULL,

   "APPLICATION#" VARCHAR2(2 BYTE) NOT NULL,

   "JOB#" VARCHAR2(4 BYTE) NOT NULL,

   "START_DATE" DATE NOT NULL,

   "EXPIRY_DATE" DATE NOT NULL,

   "ACTIVE" VARCHAR2(1 BYTE) NOT NULL

  )

/

CREATE UNIQUE INDEX GATE_USER_APPLICATION_JOB2_PK ON GATE_USER_APPLICATION_JOB2( ORAUSER, APPLICATION# )

/

ALTER TABLE GATE_USER_APPLICATION_JOB2 ADD CONSTRAINT GATE_USER_APPLICATION_JOB2_PK PRIMARY KEY (ORAUSER, APPLICATION#)