Getting comfrey seeds to sprout

I have a bag of true comfrey seeds purchased and trying to get them to sprout. I read that it might take a very long time, but its been almost 4 weeks and no progress. I did not do the freezing them before trying to sprout them ( I saved half in case this was really necessary). I have worm poop in a growing tray with a cover on a warming mat and so far no sprouts. Any advice?

Also, what is the recommended planting depth? I pulled a few out and they look like they rotted, black innards spongy and no shoots. They are a rock hard seed to begin with. I think I planted them maybe 3/4" deep in the trays so perhaps they just cooked in place instead of germinating...

I grew some from seed from Richter's last year. I remember that they said to barely cover but keep moist...so I made a small depression for each seed and could water more easily. It did take a long time and not all germinated.
I've ended up with four really nice plants that all bloomed the first year and have come back this year full force....blooming already, so I am going to let them go and try to save some seed.
If you still have seed, I don't think it is too late to start some more...they seem to grow really fast once sprouted.
Here is a bit from Horizon Herbs that sounds like how I was told to grow them....

Sow the seed just under the surface and tamp in securely. Keep warm and moist until germination, which takes approximately 10 days in standard greenhouse culture. Germination may take as long as 30 days if you do not have a greenhouse. 30 days of cold/moist conditioning followed by planting in warm soil may improve the germination response.

Thank you Judith, I think I will do just that, I have about 12 seeds left and I will pot them shallow and see how they do!

I found that some of the seeds sprouted around the comfrey(Symphytum officinale ) that I started from seed a year and a half ago. It was edged by grass and didn't receive much morning sun...see picture below.
The other comparable plant, planted from seed at the same time as the one above, received more sun but I had mulched well all around it and there were no volunteer plants.
We also had a much wetter summer than normal.
Considering the huge number of seeds that both of these plants produced, it seems like natural germination is pretty low.