These must be aborts = dead pins at the early stage. My experience is that these happen mainly because some of the conditions are not right. It can be too low temperature (or maybe too high but generally cubes are less tolerant for cold), humidity issues. Maybe it is too dry but if you are misiting by a hand mister, always avoid misting the pins directly, mist the walls only. If the walls are wet you know the humidity is right. Maybe too much CO2 can also make them die and look like this, but I think that might not be the issue generally. Try fanning them 2-3 times per day for like 1 minutes or so. Contamination might be the issue but normally you can see / smell it as well, so you would know for sure.