Hard to tell from photos, is the membrane stuck around the cotyledon?
Sometimes, you have to remove the membrane manually. If left too long on the seedling the membrane can dry and make it hard for the sprout to open. Sometimes they open on their own, but it seems to be better to get that membrane off as soon as they are above ground, otherwise you will find little brown squeezed spots that look like the thing had a turniquet on. If the membrane is there, and if it is dried/stuck to the leaves, you can drip water on to "lube it up" and with a finger nail slide the membrane off, gently.
Is there a membrane?
If no membrane it might have already damped off and died, or it might still be waiting to open (usually happens pretty fast tho, 1 or two days no open = no bueno)