I'm pretty new to all this but as far as I can gather, it sounds like a humidity issue - have you got a hygrometer? If so, what's it reading? The humidity for lockdown until all that are going to hatch have hatched should be 65% or higher. If you're sure the humidity is right, you should leave them well alone as opening the incubator (even just for a few seconds) can dramatically reduce the humidity enough to "shrink wrap" a chick who has pipped. What this basically means is that the internal membrane (there are two - one just inside the shell and then one covering the chick - it's the one covering the chick I'm referring to) can dry out incredibly quickly and if this happens, a chick will have an extremely hard time hatching and may well die trying. So long as temp and humidity are right, you should leave well alone and let nature do it's thing.
HTH