Solid core wires are generally better than jumper wires. They are more reliable and can be cut to length and laid flat to make it easy to check and find errors.
Jumper/Dupont wires are notoriously unreliable and are the first thing you should check when there's a problem.
Solid wires with a very narrow gauge can sometimes be a problem because the spring clips inside the breadboard may not make good contact, especially if the breadboard is old and worn out. Wire taken from ethernet cable can sometimes be quite narrow gauge.