Most likely explanation is they are over-heated. They need to be surface sown, have light to germinate and prefer cooler soil germination temps at around 55-60 degrees. A warm light and a heating mat, depending on the temp it is set at, could easily be cooking the seeds. There are several discussions about them over on the Perennials forum here and their suggestion is to Wintersow them for best results so you might want to check out the info on the Wintersowing forum.

Dave