You should use the ads diagnostic tool on the keyword in question. It could be that it is hot and your CPC is too low to show the ad or that it is slated for the 2nd page of ads. The information there will help you to see.
I have noticed that if you enter 100 keywords, that Google seems to only want to show about 20 or 30 keywords not to show the whole lot. I think that some of this is due to their ranking of quality score. What I do is after about 2 weeks and no impressions on those words is to just drop them from the account.
Random check on various keyphrases don't show the ads. Why not? (most of these have little competition and should show up top 5 on bid I put in)
I'm about to contact Google on this issue, but before I get started with their copy&paste bs replies, I wondered if anyone here can answer this? i.e. how to get my ads to show!