If Japanese coworkers could not understand your coworker, is it that he used complicated sentences or that he couldn't use them correctly? Being able to explain

If Japanese coworkers could not understand your coworker, is it that he used complicated sentences or that he couldn't use them correctly? Being able to explain things in simple terms is really useful, but it also means that you have to explain a lot more when conveying nuance or precision because you can't choose the word that means exactly what you want to say. Do you ever run into this, or do you feel the simple explanations can cover everything you want to communicate?