I've had some eggs sink in way early in incubation, that was usually cause of too low humidity. Even after the humidity was corrected the egg just never filled out completly again, but still went on to hatch a healthy baby. Only thing is when the eggs start dimpling nearing hatching time, those eggs that were already dimpled do start to look rather concerning cause they are sinking in a lot more, and I usually end up cutting them first to make sure the baby has adequate space.
Also about some of the eggs staying completly plump, sometimes that happens and they don't dimple at all. Tho I've only got 2 clutches under my belt, I haven't had an issue with eggs that didn't dimple.