Unable to exchange UDP packets with node.js AWS server

I would like to use my AWS instance to exchange UDP packets with various client applications.

When I run the server-side code locally, everything works as expected. However, when the code is run from AWS, I can only receive packets, not send. The logs tell me that, at least, the server-side send() is being invoked, but nothing else can be discerned.

Edit:

1. I'm not using a load balancer; I only have one instance [SO post]


2. I've enabled all UDP inbound/outbound traffic [AWS post]


3. I created a second AWS instance, and I am able to exchange packets between my, now, two instances.


4. Wireshark doesn't detect incoming packets on my client, even when its firewall is disabled.


5. I've successfully sent UDP packets to my instance (where they've been detected). The problem of outbound traffic remains.

Advice?