How long has it been since it made the first external pip on its own? Remember that it can take 48 to 72 hours to get all the way out on its own from that point, and that it is in no danger from taking too long as long as it's not showing signs of stress. On the other hand, if you help it too early it can result in a weak baby that has not absorbed the yolk fully and will not thrive as easily once out. If it is peeping strongly and there is enough of a hole for it to breathe, I'd leave it be and just watch for warning signs. The last one out is always the hardest one NOT to help, because even if it's only been 24 hours, it SEEMS much longer because you've been watching eggs for days and they all kind of run together. This is why it's important to take notes on times for each egg--so you always know how long it's been for each one.
I think chances are good that whichever way you do it, the little one will be fine. Or not, if it has something congenitally wrong, but most likely it's just experiencing the results of an unpredictable and fluctuating environment during incubation. I think your best plan of action is to relax and let it be, but everyone has to do what seems best to them. This is how we learn.
Good luck!