I read a post of a lady who on day 26 was going to throw them away and heard a pip from!! It could take longer, I'm not sure about candling them, I think it would be ok if you're quick about it.
Our babies hatched through the 23 day! We were so impatient and our runt "Squirt" had a gimpy leg and a bubble on her belly button area. It has dried up and gone away.... although, the pus would be some cause for concern. You could clean it and hope for the best. We also had to help ours out...