Quick test would be to move the junction/device at the tapping of this new conductor. When you move it, keep "Connected" option on. When you drag that junction, you will see the rubber-banding effect with existing lines. If you do not see this rubber-banding affect on your new line added, this means the line is not connected/snapped to that junction/device. Also,
1. Confirm you have assetgroup, assettype and lifecyclestatus populated correctly for the new line. Only certain assetgroup/assettype combinations and only certain life cycle states participate in the topology/tier.
2. Confirm you have a rule that allows this type of connection. You can review Junction-Edge and Edge-Junction-Edge rules and make sure the junction's assetgroup/type and new line's assetgroup/type combination is listed there.
3. Try moving the start point of the line away from the junction, add another junction snapped to the start (or end), and then establish junction to junction connectivity between existing junction and new junction added. Then run update subnetwork.
Hope this helps.