Found the solution

I added RaiseBubbleEvent method in the RaisePostBackEvent.
This method seems to forward the postback to the parent control.

Previous code
Code:
        Public Sub RaisePostBackEvent(ByVal eventArgument As String) Implements System.Web.UI.IPostBackEventHandler.RaisePostBackEvent
            OnClick(EventArgs.Empty)
        End Sub
Current code
Code:
        Public Sub RaisePostBackEvent(ByVal eventArgument As String) Implements System.Web.UI.IPostBackEventHandler.RaisePostBackEvent
            Dim varArg As CommandEventArgs = New CommandEventArgs(CommandName, CommandArgument)
            RaiseBubbleEvent(Me, varArg)
            OnClick(EventArgs.Empty)
        End Sub