Asynchronious behaves differently then blocking mode when sending data.
When you try to send data in blocking mode, while the network is busy, your command will just wait until it's possible to send the data.
In asynchronious mode you can't block (or wait), then you receive an error code (WSAEWOULDBLOCK) saying that the OS can't send the data right now.
Most people confuse this "error code" for a real problem.
The solutions are:
Wait for a OnSocketWrite event, saying you can send what you failed before.
Just try to send in a loop until you succeed. (not recommended)
In the library to indicate this situation the Send returns 0.