How Can IJ Find Lack of Credibility?

THREE MAIN REASONS
1. Lack of Detail
If the Applicant provides answers that lack specific details, dates, times, descriptions of people and what they were wearing, etc. 
This is probably the most common reason cited. 
2. Implausibility 
The story the Applicant tells seems implausible. 
3. Inconsistencies 
Testimony in Court is not consistent with written statement, I-589, or what was said at credible fear interview.   Or something you said in the beginning of your testimony does not match something you said later in your testimony. 
This is probably tied for the most common reason cited.