so just curious after they pip a hole how long before you intervene if no zipping? 24 hrs? do they dry out and stick to the shell after so long?
From what I read, if you are planning on reproducing the chicken (hatching eggs from that particular pullet), then you may not want to help it out. Personally, I think I'd help it out regardless and pray that I am not in that situation but know that anything can happen. Please remember I've never done this, like the blind leading the blind.

Someone else may have better advice but seems like people try to keep the humidity up. Maybe a few cracks to get things moving may help but you risk loosing the humidity everytime you open the incubator. I would turn off the fan prior to opening the incubator and maybe spray the walls or anywhere where it would not contact the egg to really boost up the humidity.
Please let me know what you try out and if it works. Wishing you best of luck!