A skill is something that improves with deliberate practice. In other words, if you want to see if something is a skill, ask yourself whether practice is likely to improve it. If not, then maybe you have misclassified something as a skill that is really some other kind of activity. Non-skill-like things do not tend to improve with repetition or deliberate effort.
This means that for some skills, your practice might be inadequate. For example, I have known people whose Spanish doesn't ever improve very much. You can be stuck in a plateau and just keep cooking without ever cooking anything better. In some cases there is a limit: you are pretty good at it and you might never get much better. In some cases, the quality of the practice is just not very good. Some people write a bunch and never become good writers. This is mysterious to me, but it might be that they never make a conscious effort to improve, never break down the skill analytically.