I want to replace 6 hardwired 20W 12v under cabinet lights with LED 4W 12v lights. First one replaced fine but when replaced 2 more, lights started flickering. New lights have 1 red wire and 1 black. Wires from wall are red and white. I was doing red with red and black with white. All lights are controlled by one switch. Some lights have 2 sets of wires coming from wall. Any thoughts?

See if any of these articles is helpful.

https://www.diginet.net.au/fix-led-downlights-flickering/

http://www.landlordstation.com/blog/fix-flickering-led-lights/

https://lamphq.com/flickering-led-lights/

Are they on a dimmer? Maybe a professional can check for loose connections.