I'm generating a network flood with a lot of packets coming from one fixed IP+port pair (in this example, 184.108.40.206:2003). Why do I only see one SYN_RECV'd packet in
netstat on the victim machine?
SYN_RECV is a state of a TCP socket which is identified by an IP+port pair. So all your SYN segments fall into the same socket, as the IP+port pair is fixed in your case.
As you probably don't change sequence and acknowledgment numbers as well, the TCP driver on the victim just treats all those SYN segments as retransmitted.