Or, trying to set focus to a control, on a subform, you may need to Set the focus, in stages.
First the subform, then the control,(from main form)...
Me.sfrmPayments.SetFocus
Me.sfrmPayments.Form.txtAmount.SetFocus
and, if I remember correctly, vica versa...
(from the subform)
Me.setFocus
Me.txtClient.SetFocus
Hope this helps, good Luck!