I have a written an apex class for a custom object called loan disbursement transactions. This custom object is a child object to the account. I have a currency field on the account record that calculates the sum of the loan disbursement transaction value, each of these values are coming from the child object, loan disbursement transaction. Usually there are only 2 such values. When I try to run this apex code, the total disbursements field on the account record does not update. Can someone please help me with what I am doing wrong? The apex code is below: 

public class UpdateLoantransactions{


  public static List<Loan_Disbursement_Transactions__c> listoftransactions(Account b){
  List<Loan_Disbursement_Transactions__c> transactions1 = [SELECT Name, Disbursement__c, Account__c, Loan_Disbursement_Value__c FROM Loan_Disbursement_Transactions__c WHERE Account__c =: b.Name];
 
  return transactions1;
 }
  
 public static decimal gettransactionvalue1(Account b2){
 decimal d = 0.00;
 
 if((listoftransactions(b2)).size()>0){
 d = (((listoftransactions(b2)).get(0)).Loan_Disbursement_Value__c);}
 
 return d;
 }
 
 public static decimal gettransactionvalue2(Account b3){
 decimal d1 = 0.00;
 
 if(((listoftransactions(b3)).get(1))!=null){
 d1 = (((listoftransactions(b3)).get(1)).Loan_Disbursement_Value__c);}
 return d1;
 }
 
 public static decimal gettotaldisbursements(Account b4){
 decimal d2 = 0.0;
 d2 = gettransactionvalue1(b4) + gettransactionvalue2(b4);
 b4.Total_Disbursement__c = d2;
  return b4.Total_Disbursement__c;
 }
 }

Thanks