In addition to the thread referred to above, where somebody unknowingly was rebooked in a non-earning class, there is actually one sentence in M&B's T&C that might explain this:

CHAPTER 5: AEGEAN PARTNERSHIP WITH AIRLINE COMPANIES-MEMBERS OF STAR ALLIANCE

1. (...) Individual Star Alliance member-carrier’s may also exclude mileage accrual on certain flight-ranges.

Maybe the flight you took is in some "range" that UA excluded from miles accrual. The fact that A3's representative told you this flight earns 0 no matter what booking class might indicate this. UA or A3 should be able to advise.