为什么关联锁定不起作用？

我想把lfsj和kf做成关联锁定，当kf值为no时，lfsj被锁定，不能输入，其它则可以输入，具体设置和代码如下，结果却不起作用，请教是哪里有问题，谢谢！

Private Sub Form_load()
Me.RecordSource = "Select * FROM tblssmx Where mxId = '" & selectstr & "'"
g_CurrentSelectStrID = selectstr

If Me.kf = "no" Then
    Me.lfsj.Locked = True
Else
   Me.lfsj.Locked = False
End If
 
End Sub
Private Sub cmdCancel_Click()
     Me.Undo
     DoCmd.Close acForm, Me.name
End Sub

Private Sub cmdOK_Click()
     If IsNull(Me.kf) Then
         MsgBox "请在空房打钩!", vbCritical, "提示："
         Me.kf.SetFocus
         Exit Sub
     End If

  
     Me.Refresh
     DoCmd.Echo False
     Forms!usysfrmMain!frmChild.SourceObject = "frmssmx_child"
     DoCmd.Echo True
     '触发子窗体计时器事件
     Forms!usysfrmMain!frmChild.Form.TimerInterval = 300
     DoCmd.Close acForm, Me.name
End Sub