This is not a cause for irritation. If someone does not know answer of is not interested in the topic, he will not answer. Moreover, it is better to find out answer through net search or reference books rather than asking some one.
Surely it gives a feeling of irritation as if you are unheard but then we cannot do anything about it and rather take it something that is usual in our day to day lives...
yes it is irritating especially during writing an exam and immediately u need answers, but we dont get the answer due to the hawk eyes of the invigilator !!
yes,of couse.I have got irritated many times.I would have asked a question in needy times and there would be no reply.It will sound like an insult for me.