Workflow with custom async webservices Skip some Requests

I’ve built a custom workflow activity for Sp 2010.
this one does nothing else than calling an Custom Web Services asynchronously

 private void codeActivity1_ExecuteCode(object sender, EventArgs e)
        {
            try
             {   

                LoggerObj.writeLog("Begin");  
                TaskPermissionsService Proxy = new TaskPermissionsService(); 

                IAsyncResult ar = Proxy.BeginSetPermissionsToTask(workflowProperties.ItemId.ToString(), workflowProperties.WebUrl,null,null);  
                 ar.AsyncWaitHandle.WaitOne();
                string results;     
                results = Proxy.EndSetPermissionsToTask(ar);

                LoggerObj.writeLog("End");
            }
            catch (Exception ex)
            {
                LoggerObj.writeLog("Error:" + ex.Message);
            }

Unfortunately, Reading my WebServices Logs .txt, I notice if I edit more than 20 ListItems at a time, the remaining calls does’t reach the WebServices. Completely lost!

Any idea what it could be ?