I would say that it's always worth the effort. Sometimes, people aren't real outward but they appreciate the effort that you put into the conversation.

I wouldn't continue asking questions of *Joe and Sam* if they never bother answering my questions, to begin with. Or, maybe people aren't aware that you've asked a question? Re-phrase your words, and/or speak more loudly.

I just think that if this is a regular pattern that you've noticed, then you are missing something. Make eye contact, check your voice volume and tone. Are you talking with a variety of people (and having these problems) or is it the same couple of people? If this has happened 2-3 times, that's still pretty irregular. Or, if it's just been the last week... maybe you aren't talking as loudly as you think.

Just some ideas for you to thing about.