Work on the positive association in a confined space where you're the most interesting thing going. If you have a fenced yard around the coop, that's a good spot, but if the coop is the only spot, start it in there.
As for outside- depending on what you're offering as a treat, it may be tough to compete with what they're finding on their own- but most chickens will go after a sure thing. Outside, start with your friendliest bird and show her the treats. The easiest way to attract everyone else ... is for them to see that SOMEONE ELSE is getting the treat first!
Then associate it with whatever call you want.
As mentioned above, always give them the reward. I've trained my broody hens to come to the front of the coop (where they get their own suite overnight) by associating it with watermelon or mealworms - getting their attention calling "mamma babies momma babies" didn't take long. A day maybe- much easier than trying to herd them in.
In the summer if I show up in the mid afternoon, everyone comes running - that means watermelon or scratch is up for grabs. When I toss out the watermelon wedges, it's a little creepy ... because all 140 or so will silently, expectantly jog behind me until l start dumping the pieces in the shade.
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