This is SUPER common and may be the one bit of upper-hand we've got on conversationally fluent KETs. Something like articles, which native speakers have been using since the glorious diaper, is both trivial and eternally useful in English. You can omit them, but they're mandatory in natural conversation.
My suggestion: First and foremost, only have a supplemental lesson with highly fluent students. Not only can you explain the nuances better, but they'll gain more mileage from articles. Secondly, you can begin to tackle articles when you get to count/non-count nouns as the rules with articles are directly related to the former topic. If your students have trouble with count/non-count nouns or lack the vocabulary, don't bother teaching articles IMO. You'll be opening up another can of troublesome worms.
Back home, I taught at an English language center that specialized in preparing international students for university in the States and articles are initially taught at the very beginner's levels, yet are quickly forgotten. I remember developing a lesson or two on articles and it's a tricky one. The most fluent students with the highest TOEFL scores will know on paper what requires an article, but it's the application of it in natural, unconscious language that's the true mind-boggler.
