My friend and former co-worker, Frank Belzer, used to say, "Practice doesn't make perfect. Perfect practice makes perfect!" Think about that and your job as a coach. What does your client gain by you allowing them to do anything wrong? Every time they practice something wrong, they reinforce bad behavior. Fix it before they do it or immediately after you find out that they did it.
Are you patient? If you are, stop it. You are stealing from your client. It's not about how long you can keep the engagement and how much money you can charge them. It's about how quickly and effectively you can make them better. Several years ago I knew a small business owner that paid a sales trainer tens of thousands of dollars over three years and never saw any improvement. The sales trainer was a great salesman and kept the poor guy paying, but never changed him.
Is your experience deep enough and/or broad enough? Do you know their industry so well that you're going to help them be same old - same old, just like all the others? Have you coached in other industries and can apply that experience to their world? Have you coached clients from the same age group, geography, educational level, etc.? Does it matter? Is it relevant?
Do you have the time? This is often the issue with managers. They not only have to coach, but they have several other direct reports and need to report to their own managers. They may be involved in their own selling, or actively recruiting, and have their own team and personal goals to reach. Can you put your own goals aside to focus on your client/subordinate?
Does what you feel matter? Can you let them revel in their growth and success without you taking credit for it? Can you correct, criticize, push back, knowing that they'll be uncomfortable and may even be upset with you?
Are you better off being a trainer? Standing in front of a classroom with several salespeople teaching theory, techniques, tactics and having them role play with each other rather than role playing with you and handling objections that you know that they personally need help with? Training may have it's place, but can it be expected to cause substantial personal growth?
Are you the type of person that would rather do it yourself than guide someone through it? Can you help with form and protect your client from getting hurt, but still let them stretch outside their comfort zone?
I've seen hundreds of salespeople struggle over the years and unfortunately, many of them had well meaning managers, or corporate trainers, but not personal coaches. The coach/client relationship is special and not everyone can or should do it.