Button event not fire

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C#

ASP.NET

I have two page page1 and page2 on page1 and page2 consist button with event in page1 the button event fire but after load page2 some time button event fire and some time not fire on page2.On page 2 i have used User control and that user control have button event and also use on user control MaskedEditExtender control. so i want to know in which case this type of problem occur.

My code:

protected void btnGetToalHour_Click(object sender, EventArgs e)
        {
            TimeSheetDay TSD = new TimeSheetDay();
            if (TSD.BeginingOfTime.ToString() != string.Empty && TSD.EndOfTime.ToString() != string.Empty)
            {

                if ((TSD.BeginingOfTime.Hour >= 8 && TSD.BeginingOfTime.Hour <= 17) && (TSD.EndOfTime.Hour >= 8 && TSD.EndOfTime.Hour <= 17))
                {
                    //lblMessage.Visible = false;
                    decimal StartTime = Convert.ToDecimal(TSD.BeginingOfTime.Hour.ToString() + "." + TSD.BeginingOfTime.Minute.ToString());
                    decimal EndTime = Convert.ToDecimal(TSD.EndOfTime.Hour.ToString() + "." + TSD.EndOfTime.Minute.ToString());
                    txthour.Text = Convert.ToString(EndTime - StartTime);
                }
                else
                { lblMessage.Text = "Please check entered working  strat time and end Time"; lblMessage.Visible = true; }
            }
            else
            { lblMessage.Text = "Please entered working  strat time and end Time"; lblMessage.Visible = true; }
        }