MERGE into getting stuck all of a sudden - oracle

Question

I have a stored procedure that is getting stuck on the merge into all of a sudden but has been working fine for years. Any advice ? The SP is in Oracle SQL developer and it is being executed in a Visual Studios SSIS package
Query below:
MERGE INTO CORE_APPT C
USING (
SELECT DISTINCT
TST_TKR_ID,
TST_TKR_ID_SRC,
APPT_ID_SRC,
APPT_CRETN_DTE,
APNTMT_DT,
ETS_APNTMT_ID,
ETS_TST_CNTR,
TST_STS_TYP_DESC,
APPT_STS_TYP,
TST_DLVR_MTHD_TYP_CDE,
DLVR_GRP_TYP,
TST_LNCH_TM,
TRNSCTN_ID,
TST_PKG_ID,
RND_TRIP_TST_FLG,
UPDT_DTE_TM,
PROD_ID,
ACTN_DTE_TM,
TST_DURN,
BRKRG_APPT_UPDT_DTM,
TST_APPT_TYP_CDE,
CNCL_MKUP_RSN_TYP,
DLNQNT_HLD_FLG,
TST_TKR_TST_UPDT_DTE_TM,
SCR_HLD_LVL_TYP_CDE,
SCR_CNCL_FLG,
ETS_CANDTE_SCR_HLD_FLG,
TST_CNTR_HLD_FLG,
TST_ADM_HLD_FLG,
ETS_SCR_CNCL_FLG,
TST_ADM_HLD_RSN_CDE,
TST_CNTR_HLD_RSN_CDE,
ETS_CANDTE_SCR_HLD_RSN_CDE,
ETS_SCR_CNCL_RSN_CDE,
DLNQNT_HLD_RSN_CDE,
TST_TKR_SCR_STS_TYP_CDE,
TST_TKR_SCR_STS_RSN_TYP_CDE,
PYMT_NO,
DATA_SRCE AS DATA_SRCE,
CURRENT_FLAG,
PRGRM_TYP,
sysdate as RCD_CRTD_DTE,
user as RCD_CRTD_USER,
REG_SYS_ID AS REG_SYS_ID,
TST_NAM AS TST_NAM,
CNTRY_SETL_RSP AS CNTRY_SETL_RSP
FROM STG_CMMN_APPT A
WHERE A.CURRENT_FLAG = 'Y'
) S
ON (C.APPT_ID_SRC = S.APPT_ID_SRC
AND C.REG_SYS_ID = S.REG_SYS_ID
AND C.DATA_SRCE = S.DATA_SRCE) -- added in EREG4.0 changes
WHEN MATCHED THEN UPDATE
SET C.CURRENT_FLAG = 'N'
WHERE C.CURRENT_FLAG = 'Y'
WHEN NOT MATCHED THEN INSERT
(
C.TST_TKR_ID,
C.TST_TKR_ID_SRC,
C.APPT_ID_SRC,
C.APPT_CRETN_DTE,
C.APNTMT_DT,
C.ETS_APNTMT_ID,
C.TST_CNTR_ID_SRC,
C.TST_STS_TYP_DESC,
C.APPT_STS_TYP,
C.TST_DLVR_MTHD_TYP_CDE,
C.DLVR_GRP_TYP,
C.TST_LNCH_TM,
C.TRNSCTN_ID,
C.TST_PKG_ID,
C.RND_TRIP_TST_FLG,
C.UPDT_DTE_TM,
C.PROD_ID,
C.ACTN_DTE_TM,
C.TST_DURN,
C.BRKRG_APPT_UPDT_DTM,
C.TST_APPT_TYP_CDE,
C.CNCL_MKUP_RSN_TYP,
C.DLNQNT_HLD_FLG,
C.TST_TKR_TST_UPDT_DTE_TM,
C.SCR_HLD_LVL_TYP_CDE,
C.SCR_CNCL_FLG,
C.ETS_CANDTE_SCR_HLD_FLG,
C.TST_CNTR_HLD_FLG,
C.TST_ADM_HLD_FLG,
C.ETS_SCR_CNCL_FLG,
C.TST_ADM_HLD_RSN_CDE,
C.TST_CNTR_HLD_RSN_CDE,
C.ETS_CANDTE_SCR_HLD_RSN_CDE,
C.ETS_SCR_CNCL_RSN_CDE,
C.DLNQNT_HLD_RSN_CDE,
C.TST_TKR_SCR_STS_TYP_CDE,
C.TST_TKR_SCR_STS_RSN_TYP_CDE,
C.PYMT_NO,
C.DATA_SRCE,
C.CURRENT_FLAG,
C.PGM_TYP,
C.RCD_CRTD_DTE,
C.RCD_CRTD_USER,
C.REG_SYS_ID,
C.TST_NAM,
C.CNTRY_SETL_RSP
)
VALUES
(
S.TST_TKR_ID,
S.TST_TKR_ID_SRC,
S.APPT_ID_SRC,
S.APPT_CRETN_DTE,
S.APNTMT_DT,
S.ETS_APNTMT_ID,
S.ETS_TST_CNTR,
S.TST_STS_TYP_DESC,
S.APPT_STS_TYP,
S.TST_DLVR_MTHD_TYP_CDE,
S.DLVR_GRP_TYP,
S.TST_LNCH_TM,
S.TRNSCTN_ID,
S.TST_PKG_ID,
S.RND_TRIP_TST_FLG,
S.UPDT_DTE_TM,
S.PROD_ID,
S.ACTN_DTE_TM,
S.TST_DURN,
S.BRKRG_APPT_UPDT_DTM,
S.TST_APPT_TYP_CDE,
S.CNCL_MKUP_RSN_TYP,
S.DLNQNT_HLD_FLG,
S.TST_TKR_TST_UPDT_DTE_TM,
S.SCR_HLD_LVL_TYP_CDE,
S.SCR_CNCL_FLG,
S.ETS_CANDTE_SCR_HLD_FLG,
S.TST_CNTR_HLD_FLG,
S.TST_ADM_HLD_FLG,
S.ETS_SCR_CNCL_FLG,
S.TST_ADM_HLD_RSN_CDE,
S.TST_CNTR_HLD_RSN_CDE,
S.ETS_CANDTE_SCR_HLD_RSN_CDE,
S.ETS_SCR_CNCL_RSN_CDE,
S.DLNQNT_HLD_RSN_CDE,
S.TST_TKR_SCR_STS_TYP_CDE,
S.TST_TKR_SCR_STS_RSN_TYP_CDE,
S.PYMT_NO,
S.DATA_SRCE,
S.CURRENT_FLAG,
S.PRGRM_TYP,
S.RCD_CRTD_DTE,
S.RCD_CRTD_USER,
S.REG_SYS_ID,
S.TST_NAM,
S.CNTRY_SETL_RSP
)
;
TIA!