There are, believe it or not, different types of 'shorts.' Sometimes there is enough impedance - arcing is a high-impedance type of fault - that a breaker may respond slowly.
As for the 30-amp load, the question is: how long was the load applied? Breakers trip faster the greater the overcurrent condition.
Still, everything in this world shows variations in quality, and wear out / break down with time. Maybe you have a bad breaker - but that's no reason to suspect an entire product line.
