ASP.NET Session

I have a RetriveParticipant page, which has a Onlcik even on a button with the following info:
protected void AddCollection_Click(object sender, EventArgs e)
    {
        Session["sParticipantID"] = lblParticipantID.Text;
        Session["sCollectionType"] = ddlCollectionType.SelectedValue;
        Session["sCollectionDate"] = txtCollectionDate.Text;
        Response.Redirect("RetriveSurvey.aspx", false);
    }

I am supossly storing the fields into the session.
On the RetriveSurvey.asp page, I am binding and saving a survey for each participantID.
on if (!Page.IsPostBack) // i have
        {
 lblParticipantID.Text = Session["sParticipantID"].ToString();
lblSurveyID1.Text = Session["SurveyID"].ToString();
}

On binding the survey, i am just cheking for :
 private void Bind_Survey()
    {
        if ((Session["SurveyID"] != null) && (Session["sParticipantID"]) != null)
        {
            btn_Submit.Visible = false;
            SqlCommand cmdSurvey;
            cmdSurvey = new SqlCommand("spRetrieveSurvey", myConnection);
            cmdSurvey.CommandType = CommandType.StoredProcedure;
            cmdSurvey.Parameters.Add(new SqlParameter("@ParticipantID", Session["sParticipantID"].ToString()));
            cmdSurvey.Parameters.Add(new SqlParameter("@SurveyID", Session["SurveyID"].ToString()));

adn so ..... on

on the other hand, to save a new survey I have
  protected void Save_CollectionPoint_SurveyInfo_Click()
    {
        if ((Session["sParticipant"]) != null)
        {

and so on.....


Seems like it works sometimes and it doesent in the same time.
When I click to add a new survey, it gives me the previews survey with the retrive data i had saved before. Sometimes it logs me out.
How would i manage this problem?
Let me know if yo have any other question.
Thanks in advance