WSASendTo的回调函数问题

使用的是UDP组播，overlap方式发送，调用了WSASendTo后错误代码是WSA_IO_PENDING，本机上接收端也能收到发出去的数据，但发送端就是没进入回调函数，不知为何，麻烦各位帮忙看看

socket 设置如下：
m_hSocket = WSASocket (
AF_INET,
SOCK_DGRAM,
0,
NULL,
0,
WSA_FLAG_MULTIPOINT_C_LEAF | WSA_FLAG_MULTIPOINT_D_LEAF | WSA_FLAG_OVERLAPPED) ;

if (m_hSocket == INVALID_SOCKET) {
goto JoinFail ;
}

t = TRUE ;
i = setsockopt (
m_hSocket,
SOL_SOCKET,
SO_REUSEADDR,
(char *) & t,
sizeof t
) ;
if (i == SOCKET_ERROR) {
goto JoinFail ;
}

ZeroMemory (& saddr, sizeof saddr) ;
saddr.sin_family = AF_INET ;
saddr.sin_port = usPort ; // want data on this UDP port
saddr.sin_addr.S_un.S_addr = INADDR_ANY ; // don't care about NIC we're bound to

i = bind (
m_hSocket,
(LPSOCKADDR) & saddr,
sizeof saddr
) ;
if (i == SOCKET_ERROR) {
goto JoinFail ;
}

i = setsockopt (
m_hSocket,
IPPROTO_IP,
IP_MULTICAST_TTL,
(char *) & ulTTL,
sizeof ulTTL
) ;
if (i == SOCKET_ERROR) {
goto JoinFail ;
}

i = setsockopt (
m_hSocket,
IPPROTO_IP,
IP_MULTICAST_IF,
(char *) & ulNIC,
sizeof ulNIC
) ;
if (i == SOCKET_ERROR) {
goto JoinFail ;
}
-----------------------------------------------
WSASendTo的调用如下：
LPWSAOVERLAPPED pOverlap = new WSAOVERLAPPED;
pOverlap->Internal = 0;
pOverlap->InternalHigh = 0;
pOverlap->OffsetHigh = 0;
pOverlap->Offset = 0;
pOverlap->hEvent = (HANDLE)pParam; //回调函数使用该参数

DWORD sendbytes;
i= WSASendTo(m_hSocket,pParam->wsabuf,2,&sendbytes,0,(LPSOCKADDR) & m_saddrDest,sizeof m_saddrDest,pOverlap,SendComplete);
if (i != 0) {
dw = GetLastError () ;
if (dw != WSA_IO_PENDING)
{
return HRESULT_FROM_WIN32 (dw) ;
}
}
---------------------------------------------------------------
void CALLBACK SendComplete(
IN DWORD dwError,
IN DWORD cbTransferred,
IN LPWSAOVERLAPPED lpOverlapped,
IN DWORD dwFlags)；