Forms authentication ticket seems to expire pre-maturely and kick out the user

Although I set the timeout to 60 or 1440, it seems the user gets logged out in few minutes, may be 20-30 mts.

Any suggestions would be great.HOw canI start debugging this issue?

    <sessionState mode="InProc" stateConnectionString="tcpip=something" sqlConnectionString="data source=something;Trusted_Connection=yes" cookieless="false" timeout="60" />
      <forms name=".DSA" loginUrl="frmStartLogin.aspx" defaultUrl="/frmStart.aspx" protection="All" timeout="1440" path="/" slidingExpiration="true" />
 Dim tkt As FormsAuthenticationTicket
            Dim cookiestr As String
            Dim ck As HttpCookie
            Dim intTimeoutMinutes As Integer = 1
            Dim intRtn As Integer = 1

            Try
                'set cookie timout period based on isPersistent
                If isPersistent Then
                    intTimeoutMinutes = 43200 '30 days
                Else
                    intTimeoutMinutes = 60
                End If
                'create auth cookie
                tkt = New FormsAuthenticationTicket(1, strUserName, DateTime.Now(), DateTime.Now.AddMinutes(intTimeoutMinutes), isPersistent, strUserName)
                cookiestr = FormsAuthentication.Encrypt(tkt)
                ck = New HttpCookie(FormsAuthentication.FormsCookieName(), cookiestr)
                If isPersistent Then
                    ck.Expires = tkt.Expiration
                End If
                ck.Path = FormsAuthentication.FormsCookiePath()
                'write auth cookie to client PC
                System.Web.HttpContext.Current.Response.Cookies.Add(ck)
               
            Catch ex As Exception
                intRtn = -1
            End Try