This is a good problem to have.

When you have reached the point that you can effectively use existing resources to solve questions that you can, formulate, and if you find yourself actively doing so (so it isn't just a question of thinking you can find answers, but not actually doing it), you are probably making pretty effective use of them. That is what they are for.

However your continued awareness that there are things you are missing is probably accurate as well.

At this point there are a few things which I find helpful.

1. Stretch your boundaries. While you don't have and obvious questions, there are things you probably would find useful to know about that you don't. Make a habit of identifying such areas and trying to learn more. This is useful in and of itself for obvious reasons. However it will also give you lots of specific questions, some of the answers to which will give you context for and clarification on things you thought you knew.
2. Try to answer questions. While trying to answer questions you challenge your own knowledge in a different way that tends to bring out what you only thought you knew.
3. Find out what others think is important. Every subject accumulates basic facts that experienced people all know, which are not obvious to beginners. You probably would never on your own realize that goto poses a fundamental maintainance problem, and never think of programming styles beyond procedural. If you think that there is more you should know that you don't, there is people out there who are perfectly willing to tell you what they think is important. Some of their suggestions will open your eyes.
4. Review what you think you know. How would you say it to someone else? How do you wish it had been said to you? Why does it make sense to do it this way and not that? Why did they choose the way that they did it? The answers are often enlightening...