Did the temps fluctuate a lot? What can happen when you open the incubator too long is that it can then overheat because it is rapidly heating the incubator once you close it. Avoid leaving the incubator open when candling if you did.
If temps were steady it is probably due to what MGG suggests but it could be that temps are a bit low. Take out an egg and hold it. It should feel just slightly warmer than your own body heat. It's hard to describe, come with a bit of practice but a hand check is I find a little helpful, a thermometer is best but they aren't always accurate. Our hands are very good sensors and generally you should be able to tell if an egg is a fraction below your body temp (37 deg C) or slighty above. If it feels really warm it is probably too hot.
Just a thought, in case you find it useful but avoid changing the temps any further, even just opening the incubator for half a minute will take half an hour to stabilize again so avoid opening it any more than necessary.
