IIRC, correctly, John Mark abandoned his first missionary journey.
Barnabas then took him under his wing.
IMO, the real heroes that many over look are Stephen, Barnabas, Tychicus, etc. The ones who Paul commends who served.
Barnabas then took him under his wing.
I would guess that it was a Personality conflict between him and Paul rather than anything to do with the work, and guessing is all we can do.