Fundamentals -- let's go back to the fundamentals of fundamentals (cognition)!
Cognition and agency in the real world is often hierarchical in nature. You learn a task by breaking it down into simpler tasks, and if necessary breaking down the simpler tasks into yet simpler. This is due to nothing more than algorithmic efficiency (when at all possible -- it usually is IRL), divide and conquer.
The fundamentals are the basic tasks which higher level tasks rely on. Sometimes (quite common really) the nature of this (inverted) tree is such that the higher level tasks have a sort of soft max-min relationship: your overall skill will only be about as good as your weakest subskill. An example that comes to mind is manual driving. You could be the most brilliant, strategic, high-reflex rally race car drive in the world, if you miss most of your stick shifts you will likely be a mediocre driver, if even competitive. Shifting properly and quickly makes a significant difference. So much that it's almost completely futile to practice those higher skills unless you've nailed down the basics.
When you're just having 'fun', learning something intuitively, without the sharp focus on improving, it's easy to neglect those fundamentals. They are likely areas where you have some natural relative difficulty, which can lead to shying away from them (in larger contexts sometimes this is even wise -- you want to use what you're good at afterall!) -- it could be because they're uncomfortable, painful, repetitive, boring, too difficult (break it down!) and so on. Compensating for weaknesses exists I believe, but in high levels of competition it's something extremely subtle; again risking generalizations almost every high skill individual will have fundamentals mastered.
Most of my activity is academic, and I have some anecdotes in this regard. I feel like I've really evolved when (a) I've focused on learning the basics of my field really well (going down to the math foundations and axioms) (b) focused on improving weaknesses. It wasn't intuitive to me that this attention to fundamentals could yield so much.
edit: It should be noted (as others noted) that identifying what are the fundamentals can be something difficult itself. Common tools here are reviewing your games/production/etc, or asking others (teachers, peers, etc).