The student and CFI just might not work well together. If my student busted twice that would tell me that something isn't clicking between us, and that maybe a different perspective would make something click, not necessarily that there's deficiency in my teaching or their learning ability. Also, the student might be under a lot of stress outside of training which might affect their performance during the checkride. I busted one stage check during primary training twice because my head was not in the right place due to personal issues that were going on. I probably shouldn't have been flying at that time. Outside of that stage check, I went on to get my Instrument, Commercial, CFI and II without any issues during any of the checkrides.