Why does HTTP from Illustrator plugins sometimes get blocked?

I've written several plugins that use HTTP to check for updates and to activate licenses, but sometimes the POST or GET fails. The problem is usually a firewall or HTTP blocker that needs to be turned off, but sometimes it seems nothing helps. It happens most often with Windows, but sometimes happens with Macs as well.

Is there anything I can suggest to my users to get around whatever is blocking it? One user sent me a list of steps he found to enable it in Norton LifeLock. As I understand it, the Windows firewall can be configured to allow certain incoming HTTP, but in this case, the plugin's HTTP never reaches the licensing server.

Is there anything I can do in my pugin to improve HTTP reliability? What's really confusing is that plugins that once activated normally can stop communicating and the user swears that nothing in their system has changed, yet other third-party plugins work just fine. I wonder if certain ISPs block it, but emails from my domain get through just fine. Any suggestions would be very much appreciated!

Thanks -- Rick