I've had this happen with other kinds of plants so take it for what it's worth... But every time it's happened, the seeds had either been stored for a long period of time or they were immature seeds. There is a period in healthy germination (right after the cotyledons open) where many plants can desiccate themselves and enter a period of dormancy to await better conditions if they germinated too early in the season. If you've ever germinated a bunch of seeds just for kicks and then neglected them without watering them, you'll notice how long the cotyledons will stay healthy and green looking - even after the rest of the stem/root has shriveled up - this is a practical example of the plant desiccating itself and entering dormancy to outlast bad conditions. With damaged/old/immature seeds they sometimes just don't ever leave this dormant stage or have a hard time doing so. I'd bet that's what is going on here, but it's pure conjecture.
You could try a low concentration of gibberellic acid or auxin on them to try and kick them into gear (normally this kicks a seedling out of dormancy and into self-production mode if the seeds/conditions are healthy). Many root stimulators contain auxins - might be worth a try.