The mix is unlikely. The snow is more likely. Every variety of seed has an optimal soil temperature for sprouting. You can check your soil temp with a digital meat thermometer.

Other reasons for failure might be too much moisture [seeds may rot] or too little.

What you really want to know is whether the seeds are still viable, which you can check by digging carefully into the soil.

Then you can either wait or re-seed.

Here is a link that might be useful: Seedling emergence chart

I'm also guessing too wet. If soil temps. were consistently below 40 degrees, only the spinach, kale and kohlrabi would germinate in three weeks or less.
Seeds are more likely to rot in cold soil. It may sound counter-intuitive because microbes are also slower to act, but the longer the germinating seed stays in the soil, the greater the risk of attack by pathogens.
Most of the above seeds with the exception of spinach prefer germination temperatures around 70 degrees. Spinach is best around 60 degrees. Once they have a few true leaves, they become much more tolerant of cold soils.

This post was edited by Slimy_Okra on Wed, Apr 2, 14 at 16:09

Wait, so was I not supposed to plant them yet? Everything said I could sow them outside in March. See how little I know?

I was definitely worried about the amount of rain we received over the weekend. Not much I could do about that, I guess.

I looked very closely this afternoon and saw that both my leaf lettuce and bibb lettuce seeds have sprouted! I was happy about that. :)

Every year is different. I think snow in late March is highly unusual for your zone (correct me if I'm wrong), so it's just one of those years where it'd have been better to wait. On the flip side, there are some years where spring arrives early and you can take a gamble and seed ahead of "schedule".

This March was more like February in a lot of the eastern US

Hopefully, the cold has just made your seeds slower to sprout, and all will be well in the end.

A problem I sometimes have is mice......they eat the seeds.
I'm glad some of yours have started sprouting!
I've found with my snow peas that I have to pre-sprout them in the house......that way they seem to be less palatable to the mice.

What is great about seeds such as beets, kale, kohlrabi, lettuces, and spinach is that there are typically hundreds of seeds in the packet. What I do is gamble with early plantings. If I lose the early crop, I still have enough seeds for successive crops.