In discussing tomatoes seedlings -- I like the term "helmet heads" because it's so much fun -- when they emerge trapped within the seed coat/husk/shell (I'll call it husk), I have heard several theories:
1) seeds were not sown deeply enough or were not tamped down for good soil contact in loose growing medium, because normally the seed husk is sloughed off as the sprout pulls the seed leaves out of the ground, (don't remember where I heard this -- I think here)
2) if you apply a dropper of diluted high N fertilizer, it will give the seedling the boost to grow the seed leaves out of the husk (Michael Johnson from UK)
3) putting a drop of spit on the helmet head will help to soften the husk (someone here said that)
4) I've also tried water, dilute AACT, UCG (used coffee grounds) water, and soaked alfalfa pellet water
In addition to those that grow full sized seed leaves while "holding hands" -- seed leaves connected by the seed husk -- but otherwise healthy seedlings, I currently have/had a few that remained helmet heads without the seed leaves ever emerging, some that damped off at the bottom of the husk, some that the husks eventually softened and came off but the seed leaves never grew more than about 1/4", some that have grown very slowly beyond the stubby seed leaves to grow true leaf/leaves.
Once the true leaf/leaves grow, some will grow on and are fine, others continue to grow but slowly.
But rarely some then develop deformed single true leaf without further growing point. I have also had abnormal seedlings in the past that grow what appears to be huge, thickened seed leaves, maybe a single deformed true leaf -- I have at least one that looks like this now. I have heard that sometimes this kind of deformed seedlings can stay that way for months without ever growing anything more.
Aside from the last mentioned deformed seedlings with fatal flaw, I think the key is for the seedlings to germinate and sprout with vigor.
IMHO I think what can contribute to helmet heads are:
1) older or imperfectly stored, less vigorous seeds
2) cooler soil temps
3) letting the growing medium become too dry or too wet
4) too low humidity when the seed husk is lifted from the soil mix, drying it out before the seed leaves can swell to open it
5) complete lack of nutrients, including neighbor seedlings robbing the slower emerging seedlings in community sowing
6) --this one is a working theory-- severe fungus gnat infestation resulted in gnat maggots eating away the root hairs of newly germinated seeds before it could sprout
I haven't tried this with consistency myself, but some people have reported success from soaking the seeds before sowing when sowing older seeds with lower germination rates. Up to 1/2 hour in dilute solution of chlorine bleach, non-chlorine bleach, or a chemical that I can't remember (acronym of TSP or something) have been mentioned. These have to be rinsed off thoroughly before sowing. Also (weak?) black tea and full strength chamomile tea....