Why would a loop-back plug not work on some NIC's?

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Hello ITPRO.TV Forum!
I was watching the "Network Media Part I" and decided to make my first loop-back cable. I do have experience making my own cables but I thought this would be good to have and fun to make.

I created the plug and tested it with success. I crimped Wire 1 to wire 3 and wire 2 to wire 6 and it worked the non-switch port, what is that port called? It's the port that connects the patch cable to the wall. Anyway it worked on that port.

I tried it on a few work stations with success. However on one of my identical machines it did not register, but when I put the patch cable in connected to the switch it comes online. Is there any reason why the NIC wouldn't respond? It's not damaged because it can connect with the patch cable to the switch, and it's not the plug because the plug works on other devices.

It's not super critical but would anyone know why that is?

jcushing,
Great job putting theory into practice. Troubleshooting is your key step here.
You've already starting the pin pointing.
1. Your previous connections have worked with working computers.
2. Your new connection is not working with your new computer.
You need to think about what is working vs what isn't working.
Test your computer that isn't working by connecting to a known good connection. if this works, the computer NIC is the issue. Verify you're getting link and activity lights. You could go as far to check if you're getting incoming traffic but no outgoing traffic is happening or vice versa (you can see this on the Local Area Connection Status). If you're getting unidirectional traffic, it may indicate a bad connection; if you're not getting traffic, in or out, it may indicate your NIC.
Test the connection by connecting a previously working computer to the connection that may not be working, if this computer no longer works, then the connection is the issue.
I would at this point, assume your cable is the issue. Handmade cables are problematic, it doesn't take much for one of the connections out of 4 that are needed to not get deep enough into the connector.
Cordially,
Ronnie Wong
Host, ITProTV

Thanks for the response. Looks like it is officially something the adapter cards and or settings in these machines.
Since talking to you I created 3 more cables all of which work fine on cisco routers and switches (Over ten were tested). On desktops only some NIC cards have the issue and others do not.
I was messing with some of the auto-sense settings on the adapter in Windows on a working machine and was able to get the card to stop sensing the loopback plug, however when I turned the setting back on in never came back on. I attribute this to Windows being Windows.
I don't ever think I would need a loopback plug for a desktop or laptop NIC but now I know they won't always register because on NIC card settings.

Jcushing,
Thanks for letting us know!
Cordially,
Ronnie Wong
Host, ITProTV