First of all let me tell you one thing that if you are using validation group then all the controls of that group will be validated. If validation is done and result is true then only the server side code will be fire. Check for your controls in that group. If any of the controls is giving error means the server side code will not be fired.
Just remove the validation group from your button. Try this:
<asp:Button ID="btnNext" runat="server" Text="Next Step" SkinID="Button"
onclick="btnNext_Click" CausesValidation="False" />