Startups are really bad about this. I'm 1 for 4 on getting replies from YC companies recently. One is leaving me hanging after a full day coding project + an interview that (I think!) went really well.

Just gotta keep trying. It's a numbers game.

Having been on the other side of the equation and been bad about keeping tabs on applicants in addition to my normal duties, I can't help but feel it's somewhat karma-related, too. :) In small companies, I figure it's generally due to not having people dedicated to sourcing/hiring so things just get overlooked.