Here's a senario:

Got through INDOC, ground school and CPT great.....got to the sim and struggled. Sent home and told you'd get 2 extra sessions to straighten things out in a week or two. Sit at home for 3 weeks then called by the Chief Pilot and told they no longer wanted to offer the extra sim sessions and gives you the option to resign.

Do you explain this in a interview just like it happened or word it differently? Technically, it's not a 121 failure BUT it shows you couldn't finish the program.