Microgreens - uneven growth??

I'm new to microgreens.  I'm sprouting these guys in a dark box near the fireplace to keep them warm.  Misting with water twice daily.

For now the soil consists of:
33% Miracle Grow Potting mix (not the most organic thing I know, but still at least has nutrients)
33% Bark chips
33% Perlite
... and 1% math :)-

Seeds are brand new from Mountain Valley and regardless of species I seem to get this result.  Some sprouts are very tall -- almost ready for harvest.  Meanwhile others have not yet sprouted.

From all the stuff I've seen online they're supposed to look more or less like a nice, even lawn of sprouts.

Any idea what I'm doing wrong?

How are you sowing the seeds? Direct on top or covering with soil? The only microgreen I would cover with soil is field peas from my experience growing them for local restaurants while I was in university. I would suggest ditching the wood chips, or just try something like coco coir for the soil medium. My best success has come from evenly distributing on top of the medium, not too many seeds as overcrowding will stunt germination rates, and try not too over water. I use a bottom watering tray system and like to water every two days. Also the long spindly thin growth on new sprouts is in my opinion a sign of not receiving enough light, this often happens to me when I forget to remove the dark box cover on them and they spend a day or two too long in darkness as well. They will still grow in low light or a window sill, but for lawn like coverage I believe they could use more light. Microgreens are so much fun let me know if I can help more! Below are some examples of the micros I have done in the past.

Are you soaking the seeds prior to sowing? If not, doing so might help get more even germination.

I stopped bothering with soil at all. Plants are using the nutrition from the seed for th first little bit anyway. I don't notice a differencebetween sunflowers or peas grown in soil or out. If my greens are slightly less nutritious grown out of soil, I'm okay with that considering how much easier and cleaner it is. I'll just eat more to make up for it ;)

I would also ditch the bark chips.

Regarding process: Water the seeds overnight, then strain. With a spoon spread them out evenly onto the previously (!) watered and flattened soil.
According to Peter Burke's book, you could then cover the seeds with soaked and drained newspaper, folded to the required surface.
Dual purpose: You keep the soil/seeds constantly humid without the need to bother with them and at the same time you put some weight on them which results in stronger growth (i.e. thicker stems).
Only when the stems are fairly tall and pushed up the newspaper, you place them in the light. They will go from yellowish to green within a day or so.

I also have that seedbox with three stories. I like it for sprouting wheat. But for real greens, I prefer the soil method as you can actually "harvest" them without the roots. I acknowledge it is a bit less clean and easy though.