could you try defining the "CheckedChanged" method inline with the checkbox control inside the repeater:
<asp:CheckBox ID="tag" runat="server" AutoPostBack="true" Text='' CheckedChanged="checkbox_Changed" />
Then you don't have to worry about binding the handler it in code behind. If you're not getting the correct state of the checkbox, also make sure that you have "EnableViewState" on the page properties.
I hope this is helpful, let me know if I've misunderstood the question