Dont worry about it. Your not pinching work even if they had another carpet cleaner before.
No one owns customers. They make a choice
If any one feels threatened by other carpet cleaners doing the job cheaper then they are going after the wrong customers.
With regard to your original question not everything comes out. However as has been stated get some training in. I come from a window cleaning background and still have a window cleaning round I have found carpet cleaning is not something which can be picked up like window cleaning. Courses are not that expensive and you will learn more than you think. Training gives you confidence, gives you a structured way of tackling problems, and you will have much better results meaning more referrals and more business.