The chicks may have been too old. It works best to foster chicks when they're less than 24 hours old (2 days old at the most), as they're most likely to imprint on the hen at that age. If she was a proven broody, she may have moved right into "mommy hen mode", and began clucking to them and brooding them, but if she's had no former experience, it explains why she ignored them.
Based on your description, I think I'd put some eggs from the incubator under her. During the 24 hours or so prior to hatching, there's 2-way communication between the hen and her unhatched chicks. The chicks chirp within the eggs, and the hen should periodically answer back. This gives them time to get to know each other and also prompts a shift in the hen's hormones and behavior -- from incubating mode to hatching/brooding mode. Perhaps if she's able to hear this peeping and hatch them out herself, she'll accept the chicks.
Best of luck to you, and please let us know what happens.