Our average last frost date is now officially Feb 13th. So I missed that window. My inclination is to still pay attention to the old frost date which was May 15th as we can have 'blue northers' at any time of the year where the temperature can drop 40 degrees or more in just a few hours. This year there is ever indication that we've already had our last frost. My apple tree started blooming a couple weeks ago.
Even if I'm wrong, I'm probably not hurting the plant. I'll pull them after a few days and then check the paper towel daily until the tiniest hint of
root pokes out and then plant them all in the ground. It works well with peas and seems to be working with parsnips and pistachios. Seems worth trying with other letters of the alphabet, too.
If this does work, I'll plant any seed that I save in late January next year. I had been planting several weeks after the last average frost date, which means even longer after the last actual frost. If they need cooler temperatures to germinate that would explain what was going wrong. I know in the late summer when it will be time to start plants for fall it is often recommended to prechill the seeds of things like lettuce and broccoli, for just this reason.