Why getting time out on executing below query in SQL?

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C#3.5

I am getting time out on executing below query(SP)

 SELECT '5051'                               
 , ROW_NUMBER() OVER(ORDER BY DDRNextSenddate) as SponsoringBankSequenceNo                                
 , 'AUTH' as InstructionCode                                
 , MAINT.OriginatorIdentificationCode                                
 , MAINT.MandateReference                                
 , dbo.GetDirectDebitReferenceNumber(MAINT.MandateReference)                                   
 , 'AED' as CurrencyCode                                
 , MAINT.DirectDebitMinimumAmount as ClaimedAmount                    
 --, CASE WHEN ISNULL(OMAIN.AccountNowithBank,'') != ''                 
 --    THEN dbo.DDSDecrypt(OMAIN.AccountNowithBank)                 
 --    ELSE dbo.DDSDecrypt(MAINT.SettlmentAccount) END AS OriginatorAccountNumber                                
 , BNK.RoutingCode as PayingBank                                
 , dbo.DDSDecrypt(MAINT.Accountnumber) as Payer                                
-- , @Department                               
-- , @ip_CREATEDBY                  
 , Getdate()                 
 , 0                
 , 0                               
 ,GetDate()        
 ,ISNULL(MAINT.DDAType,'F')       
 ,NextDueDate                               
 FROM DDS_DDAMaintenance MAINT                                
 INNER JOIN DDS_Banks BNK ON BNK.EID=MAINT.PayingBankID                          
 INNER JOIN DDS_ORIGINATOR_MAINTANANCE OMAIN ON OMAIN.originatoridentificationcode=MAINT.originatoridentificationcode  

select COUNT(1) from DDS_DDAMaintenance--78455 RECORDS

select COUNT(1) from DDS_Banks--159 RECORDS

select COUNT(1) from DDS_ORIGINATOR_MAINTANANCE--18 RECORDS
While fetching from DDS_DDAMaintenance it takes only 3 sec
select * from DDS_DDAMaintenance--(3 SEC)

But on joining with DDS_Banks & DDS_ORIGINATOR_MAINTANANCE the execution time is quite high and getting timed out.

What I have tried:

select COUNT(1) from DDS_DDAMaintenance--78455 RECORDS

select COUNT(1) from DDS_Banks--159 RECORDS

select COUNT(1) from DDS_ORIGINATOR_MAINTANANCE--18 RECORDS
While fetching from DDS_DDAMaintenance it takes only 3 sec
select * from DDS_DDAMaintenance--(3 SEC)

But on joining with DDS_Banks & DDS_ORIGINATOR_MAINTANANCE the execution time is quite high and getting timed out.