Databinding + Tab controls problem

in my form, there is tab page 1 and tab page2, I use databinding to bind all
the textbox.
1) user press "new" button, bmMyInvoice.addnew()
2) user do some calcluation in page1, e.g txtInvTotalAmt = txtNoOfUnit.text
* txtUnitPrice.text
3) user click to page2 , and input another information.
4) user click back to page1, he found the "txtInvTotalAmt" gone blank. WHY ?
or If the user didn't click to page2, He just minize the form. and then max.
"txtInvTotalAmt" gone blank TOO
My conculsion is " every textbox which is not "input manually", it will gone
blank
More, user input "USD" currency, the textbox "txtCurrencyRate.Text = 1" will
be run.
This textbox txtCurrencyRate gone blank TOO , why ??