Assuming the exact same methods were used on these vs. all your other starts then I tend to blame the seeds. If these were the only seeds started in the peat pellets then there could be several other causes due to using the pellets.

On the other hand, 12 days isn't all that long when it comes to germination as there are so many factors involved. So I'd keep them well watered - the peat pellets tend to develop dry pockets deep in the middle even while the outside looks wet - and give them more time before tossing them.

Dave

Thanks, digdirt. I think I'll wait a few more days and then try again.