The bulb holder has a pair of tangs. It drops into the hole and you rotate it to lock it in place. As you do so those two tangs contact the printed circuit (dash lights power on one side, ground on the other)
You have poor contact there. Pull the bulb holder out, and with a bit of emery or sandpaper clean the tangs on the bulb holder. Also inspect the printed circuit, make sure it's clean and not corroded where the bulb holder goes in.
Rene