Nope, it's not bugged, it's just bad luck. Well, the drop chance is really low on T1/T2 as well (25%/28%) - join a community for key farming to kill him on a higher level. But honestly, the Hellfire Ring is incredibly hard to get and totally not worth it, don't waste your time unless it's "for fun" - but it doesn't sound like fun what you're experiencing.
For all other RNG discussions (because it's just RNG in the end):
http://eu.battle.net/d3/en/profile/Time2Kill-1230/hero/23330792
Nope, it's not bugged, it's just bad luck. Well, the drop chance is really low on T1/T2 as well (25%/28%) - join a community for key farming to kill him on a higher level. But honestly, the Hellfire Ring is incredibly hard to get and totally not worth it, don't waste your time unless it's "for fun" - but it doesn't sound like fun what you're experiencing.
For all other RNG discussions (because it's just RNG in the end):
http://www.diablofans.com/forums/diablo-iii-general-forums/diablo-iii-general-discussion/88935-the-rng-thread
Continue discussion there - thread will be locked.