Key up event of a textbox is not working

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C#

hi dere,

i am working in winapp.Have taken 2 textbox with 1 dropdown n 1 datetime picker.I have to move from 1 control to another by pressing enter.My first control is Paymode(dropdown),2nd one is PayDate(datetimepicker),then 3rd one is PayAmount(Textbox) and fourth one is Cred/CheqNo(Textbox).

so while i choosing cash in paymode its moving perfectly in first time,bt when m choosing credit in paymode its nt moving to the next control in second time.sometimes its stuking near date control and sometimes near payamount.

it was working perfectly before,but not working now.have already checked the key preview property of the form..what would be the problem i dont understand.Please help me to solve this..

i am sending you the code,

private void txtPayAmnt_KeyUp(object sender, KeyEventArgs e)
       {
           if (e.KeyCode == Keys.Enter)
           {
               if (cmbPayMode.SelectedIndex == 0)
               {
                   MessageBox.Show("Please  Select  the  PayMode..", "Restaurant", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                   cmbPayMode.Focus();
                   if (cmbPayMode.Focus())
                   {
                       cmbPayMode.FlatStyle = FlatStyle.Popup;
                       cmbPayMode.BackColor = System.Drawing.Color.Brown;
                       cmbPayMode.ForeColor = System.Drawing.Color.White;
                   }
               }
               else if (cmbPayMode.SelectedIndex != 1)
               {
                  txtCredChq.Focus();
               }
               else
               {
                   int j = dgvPayDetails.CurrentRow.Index;
                   dgvPayDetails.Rows[j].Cells[0].Value = cmbPayMode.Text;
                   dgvPayDetails.Rows[j].Cells[1].Value = txtPayAmnt.Text;
                   dgvPayDetails.Rows[j].Cells[2].Value = txtCredChq.Text;
                   dgvPayDetails.Rows[j].Cells[3].Value = dtpPayDt.Text;
                   dgvPayDetails.Rows.Add();
                   dgvPayDetails.CurrentCell = dgvPayDetails.Rows[j + 1].Cells[0];
                   cmbPayMode.SelectedIndex = 0;
                   txtPayAmnt.Text = "";
                   txtCredChq.Text = "";
               }
           }
       }

Thanks..