Hero armor.

Is there a trick to getting the Hero armor pieces? Apparently there is a chance each time I kill a guild lord/portal mage/crazy angry spirit. The thing is I never get them. Rather, the first time I ran any challenge mission I got piece. No matter how many times I've run since then, I never get them anymore. What gives? Is there some special requirement I am not meeting? Thanks..

Yes, all armor pieces appear in your inventory as random rewards for killing specific creatures per challenge mission...

Stolen Sunspear Armor appears in your inventory at random per Corsair Guild Lord that you kill, says more in the Guildwiki here.

Ancient Armor Remnants, same basic acquisition as above, more info here.

Mysterious Armor Pieces, same basic acquisition as above, more info here.

Primeval Armor Remnants, same basic acquisition as above, more info here.

That should help answer your questions.
cheers!

Wierd, after posting this I got a Stolen Sunspear piece each time for 5 runs in a row. 3 months of nothing and then 5 in a row...

I've spent MONTHS trying for pieces. Got one the entire time. Spent the afternoon trying, lots of folks got them 2 or 3 at a time, and I still have ONE. Booo, poor idea ANet. At least make it so you get faction that you can turn in for armor pieces just for doing those crappy challenge missions.

I got five of each (except Primeval). It took quite a few hours in each challenge mission, but I got them in the end. Ancient remnants were the easiest, altough it took quite some time.

You can usually find people selling them, if you have trouble getting any. The prices are quite high, though.