Coordination....hmmmmm, what does one mean by coordination? Are you saying they can't dance to the music? If this is the case then I say no big surprise. You are taking something (movement) that is controlled mainly by the subconscious brain and putting it into conscious awareness. The more things you bring the person's attention to the more he or she has to consciously consider and put into action.
Now that I've said that let me tell you why it applies. The active part of the conscious brain is called the working memory. The average working memory can handle 7 chunks of informtion at once. For many that's an overestimation and they can handle only five or six. This does not include any "automated" or subconscious tasks. Your body can handle hundreds of those simultaneously. You as their dance instructor are taking automated tasks and putting them into working memory. You ask them to learn the step. The first time or ten through all they are capable of thinking of is that step if that step includes less than four discrete movements. Perhaps they can pay attention to even less than that if its their first day, let me illustrate by counting what might be in thier working memory.
1. Okay I have to keep my shoulders back and down.
2. My butt isn't where it's supposed to be but where did he tell me to put my butt?
3. I don't want to step on her toes.
4. What direction is my head pointed?
5. Am I looking like an idiot? Is someone looking at me?
6. Was I supposed to start with the right or the left?
7. Right foot, and now my working memory has been completely taken up and I don't even remember what the hell step this is.
So that's with someone who has a full working memory. What you are doing is trying to automate the processes so the conscious brain doesn't need to get involved. So then you make the frame and body positioning automatic, then the head direction, how LOD works. Then you can start adding the finesse of heel and toe leads and arms and deeper dimensions to your dancing. The steps eventually become chunks of knowledge that can be strung together and applied to music taking up only 1 part of the working memory, musicality becomes the 2nd consideration, 3rd becomes floorcraft, 4th becomes how his partner is following, then if he is competing he still has room to consider performance.
That is the answer from neural-science and even if it doesn't answer your specific question it does at least underline the absolute necessity of practice.