Is it in a small egg? That is the first thing I thought of. I have set several smaller eggs and sometimes they hatch and sometimes they do not. I assume it has pipped internally but not externally since you hear it peeping. Maybe it just can't get turned around to pip externally due to a small egg. I do not ever help if it does not pip externally. This one may just be behind a couple days also. Make sure you keep the humidity up 65% to 70%. I have read other pip the egg in situtations like yours but this is not something I do. I think it weakens the flock and eventually you will be raising chickens that can not hatch themselves. I sure hope it hatches 100% would be great. Keeping my finger crossed for you and the chick!
Hope I have helped.
Tisha