For a dark room: look for any source of light, and then see if there is a way to block it. For example, close the curtains in a bedroom, close the door, turn out the light, then go into the closet and close that door too.
For a brighter light: make sure the battery is fully charged. I know some...
Is there any way to candle in a darker place, with a brighter light? Sometimes that helps.
If you still can't see any veins, I would probably give them a bit longer to see if anything changes. Eggs with thick or dark shells can be hard to candle, but at later stages you can sometimes see when...