I've found that calling them over and tapping the nipple/drinker and letting some water drip out can entice them enough to try it.
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one more thing, it took me a while to figure out that mine were actually getting water. they don't drink from the nipple per say, it's more like they nuzzle it and water, because of surface tension, runs almost imperceptibly down the trigger and around their beaks and into their mouths. it takes more work than drinking from a bowl. the nipples are red because even though chickens have terrible night vision they have amazing color vision and the red typically becomes irresistible. after reading "animals make us human" by temple grandin, I embraced the notion that chickens benefit from repetitive things to do, particularly with their beaks, particularly if they are cooped up. I think of the nipple waterer as a type of fascinator that occupies their time, yes it takes more work to get water, but they have lots of time on their "hands" so it ends up being an activity.