I put ends on the raw cable and have done some poorly. That can cause errors or no connection at all. I have a tester though. I have found that wiggling the cable end and keeping it straight with no stress can help expose a bad crimp job. I usually cut those connectors off and redo it. In your case sounds like they are pre-made so look at both ends of the long piece to see if the 8 wires are in the connector. Hard to tell entirely I know. If you have some stress on the ends also remove that and retry. You do not need to straighten the entire length. Just take stress of the connector on either end, You might see improvement. A network tool might also tell you if errors are being seen. You might see errors when your long is used and none when your short is used. Might need to replace the ends on the long cable to salvage it.