But to answer your question, they can get stuck once they start to zip, if they pause too long. I just completed an assist on a chick that zipped a third around, then got rolled by another chick and stopped. The membrane was stuck to its wing like glue!
So if they don't complete the zip within about an hour or so, or if they appear stalled out, they could need assistance. Usually just moistening the membrane with some Vaseline or antibiotic ointment (no pain killer in it!) will get the membrane unstuck enough for them to push out on their own. It may be necessary to remove the whole air cell cap, and even peel off some of the membrane. Then let them do the rest on their own. Just make sure the bill or beak is clear.
And be cautious of blood vessels. Any active bleeding, stop the bleeding and stop proceeding.