You cut at a 45 degree angle to expose more of the soft tissue where there are undifferentiated cells that can easily be coaxed into roots.
When you put your clones under the dome, don't crowd them. They should not be clumped too closely together. They need plenty of space and air flow. Not like, put a fan on them, but like, when you open the dome twice a day, not to trap stale air among the plants.