The most likely reason why a UDP non-blocking send would fail is that the UDP socket's in-kernel outgoing-data buffer is full. In this case, send()/sendto() would return -1 and errno would be set to EWOULDBLOCK.
Note that a non-blocking send()/sendto() doesn't actually send the data out the network device before it returns; rather it copies the data into an in-kernel buffer and returns immediately, and thereafter it is the kernel's responsibility to move that data out to the network as quickly as it can. The outgoing-data buffer can become full if your program tries to send a lot of data at once, because the CPU can add your new data to the buffer much faster than the network hardware can forward the buffer's data out to the network.
If you get a -1/EWOULDBLOCK error, usually the most graceful way to handle it is to stop trying to send on that socket until the socket select()'s (or poll()'s, or etc) as ready-for-write. When that happens, you know that the in-kernel buffer has been at least partially drained, and you can try the send()/sendto() call again.
Another (less likely) cause of an error from send() would be if the IP address you are trying to send to is invalid. In any case, you should check errno and find out what the errno value is, as that will give you better insight into what is going wrong.
Btw the behavior described above is not unique to UDP, either; you can and will have the same problem with a non-blocking TCP socket (even if the remote peer's receive window is not full) if you try to send() data on the socket faster than the local network card can drain the socket's in-kernel buffer.
