Large games are complicated, and developers often don't have access to all of the hardware the game will end up running on, nor time to test all possible configurations even if they did. For smaller projects it's relatively easy to ensure your code is bug-free, and there are methods to help find and eliminate bugs, but as software gets larger and more complicated it becomes easier for small errors to slip in unnoticed.
In some cases errors may also come from outside your otherwise correct code -- perhaps a library you're using has a flawed implementation on some particular platform, or in certain situations requires some special handling that you weren't aware of.
Working with multiple developers -- especially as team sizes increase -- can also often increase the risk of problems occurring unless carefully managed with clear communication, good/clear division of responsibilities, accurate and up-to-date documentation, and appropriate use of tools such as source control. If some code you write is confusing or does things in an unusual or unexpected way you may not have problems using it yourself, but when another developer has to use that same code there's suddenly the potential for mistakes to be made.
Some strategies include always initialising variables on creation, being vigilant with error checking and usage of assertions (or similar mechanism for your language), turning up your compiler's error level to the highest/harshest setting (and turning on "treat warnings as errors" if available as a separate option), and various forms of automated or semi-automated testing; a good starting point might be to research "unit testing" and "regression testing".
You should also take due care when selecting libraries or SDKs. Ensure your target platforms are properly supported, be sure you understand correct usage, and try to always use a stable release whenever possible for any important code -- be sure to take extra care if this isn't possible. More popular and well established APIs will (usually) be both better tested and better supported than something newly developed.
I want to reach a point where my knowledge in game programming is sufficient.
Sufficient for what?
Unless you have some specific end-goal in mind, as long as you're able to accomplish the next task on your to-do list you can consider your knowledge sufficient. Ideally, you should be able to create a full game (including menus, audio, victory screens, etc.) without having to rely completely on tutorials or "cut & paste coding". It's fine -- and perfectly normal -- to use reference materials and to occasionally check out a tutorial on some new technique or an area you want to improve, but you should be solving problems for yourself and using plenty of your own ideas.
You should have a solid grasp on your language of choice, be reasonably familiar with your chosen tools (including debugger, which is a very valuable tool!) and if applicable (i.e. if you're not using some non-extensible development package) be familiar with the use of additional libraries -- you will probably also have at least some level of familiarity with one or more chosen libraries, although more important than this is the ability to learn a new library if/when required.
If your game contains all of the things you listed then it sounds like you're doing just fine; if it doesn't already include it you might also consider adding audio, otherwise you've checked all of the main boxes for learning about development with your game. If you want to learn more of the full process you may also consider releasing your game in some way -- even if it's just as freeware -- so that you can learn a bit about deployment, marketing, support, etc. Other than that, just continue working on your game until you're happy that there are no more features you really want to add, and then move on to whatever your next idea is.
Hope that's helpful!