550B's #1 cause of failure-to-prime.....
....the operator forgetting to push forward on the handle.
Missing a primer can create quite a mess. I make it a habit to pay attention to primer seating. You can feel, during seating, if a primer was missed by the primer slide. But... I haven't had either of my primer slides fail to pick up a primer in a very
long time (even with sharp-edged CCI primers).