So what about those search terms that you are trying to target but aren't showing up in your search term logs? The first thing you'll want to do is run a search for your targeted term and see how far down the list you are. Have a look at the look at what your competition is for that particular term. Maybe the competition is stiff, maybe you haven't targeted that term well with any of your pages.
If you have stiff competition, you should think for a minute before you spend hours that turn into days trying to improve your position for that term. Perhaps you can better spend that time reaching the top for other less popular terms. Spending all your time targeting one big term is very much putting all of your eggs in one basket. Don't forget that if you're working hard to get a popular search term, there are many others working at least as hard as you are and your position will likely fluctuate wildly. Spreading your work around drawing less traffic from more sources will likely net you the same volume and be more resilient towards a drop for any particular term. This will likely serve you better over the long term.
Now we have a more challenging question to answer. What traffic are you missing out on? To answer this, we need to find out what people are searching for. Overture has a keyword suggestion tool that can give you an idea of what some people are searching for. One thing to note here is that Overture is a pay per click search engine so the people searching tend to be business to business searchers so their numbers may not match other search engine's numbers. But this still should give you some ideas as to what people might be searching for. If you don't have pages targeted towards the major derivative terms.