Below is my code to change the Request Field value to after deleting. It works fine, but not all the time. When i delete primary request, the secondary request must be changed to primary and the tertiary must change to secondary. 
Sometimes when i delete primary, tertiary is changed to secondary but secondary is not changing to primary. Any idea please.

List<Reclassification_Request__c> lstSecondaryReq  =[Select Id,Name,Request__c, MGCRB_CBSA_Code__c from Reclassification_Request__c where Request__c='Secondary Request' LIMIT 1];
       List<Reclassification_Request__c> lstTertiaryReq  =[Select Id,Name,Request__c, MGCRB_CBSA_Code__c from Reclassification_Request__c where Request__c='Tertiary Request' LIMIT 1];
       For(Reclassification_Request__c RR : Trigger.old) {
       If (RR.Request__c == 'Primary Request') 
            {                                
                    If (lstSecondaryReq != null &&  lstSecondaryReq.size() > 0) {
                    Reclassification_Request__c SecondaryReq = new Reclassification_Request__c();    
                    SecondaryReq = lstSecondaryReq[0];
                    System.debug('Current Value To Modified '+SecondaryReq.Request__c);
                    SecondaryReq.Request__c = 'Primary Request';
                    update SecondaryReq ;
                    System.Debug('Primary to Secondary Update Completed'+SecondaryReq.Request__c);
                }
            
                If (lstTertiaryReq!= null && lstTertiaryReq.size() > 0) {
                Reclassification_Request__c TertiaryReq = new Reclassification_Request__c();    
                TertiaryReq = lstTertiaryReq[0];
                TertiaryReq.Request__c = 'Secondary Request';
                update TertiaryReq;
            }
     } else if (RR.Request__c == 'Secondary Request') 
            {
                If (lstTertiaryReq!= null && lstTertiaryReq.size() > 0) {
                Reclassification_Request__c TertiaryReq = new Reclassification_Request__c();    
                TertiaryReq = lstTertiaryReq[0];
                TertiaryReq.Request__c = 'Secondary Request';
                update TertiaryReq;
            }            
     }
       }