Ado.net Series Quiz 2 about distributed transactions

ado| Distributed issues:
For examples of distributed transactions:

Database A and database B each have a bank table (in fact, A and b on different machines, different types of database (Support DTC))
Table structure is as follows
BankAccount, Amount

A record in the database:
A 1000

B data in the database
B 1000

For easy testing, COM + is not used, but is tested directly in the Web service.
The code is as follows:

<webmethod (Transactionoption:=enterpriseservices.transactionoption.requiresnew) > _
Public Function dtstest () as String
Try
Contextutil.enablecommit ()

Drawmoneyfroma (100)
Depostmoneytob (100)
ContextUtil.SetComplete ()
Return "ok!"
Catch ex as Exception
ContextUtil.SetAbort ()
Return "Failed" + ex. Message
End Try


End Function


Public Sub Drawmoneyfroma (ByVal amount as Long)
' Extract a certain amount of money from A
Dim CNN as New SqlConnection ("Server=server1;database=a;uid=sa;pwd=password@1;enlist=false")
Cnn. Open ()
Dim cmd as New SqlCommand ("Update Bank set Amount=amount-" & Amount & "where bankaccount= ' A '", CNN)
Cmd. ExecuteNonQuery ()

End Sub

Public Sub Depostmoneytob (ByVal amount as Long)
' Add a certain amount of money to the B account
Dim CNN as New SqlConnection ("Server=server2;database=b;pwd=password@1;uid=sa;enlist=false")
Cnn. Open ()
Dim cmd as New SqlCommand ("Update Bank set Amount=amount +" & Amount & "where bankaccount= ' B", CNN)
Throw New Exception ("FF")
Cmd. ExecuteNonQuery ()
End Sub


The Web service was then invoked, and the balance of a was found to be 900, while B's account was still 1000, and the distributed transaction failed.

What are the possible causes?

The same code as above can only be modified slightly.