There is more to it than just following the course material. Different topic, market/niche, ability to connect with potential buyers, having sufficient capital for say ads, ability to write well, expertise, etc. There are quite a lot of variables that can mean the difference between earning a few dollars each month, and having substantial side income.

I say this because I'm earning money online via affiliate marketing, own products, etc. now. It is not as easy as just follow the course. I was tempted to drop money into courses in the past, but at the end, I didn't. Not to say a course is not helpful.

Besides that, some times what people teach may not be that effective by the time you apply it, so sometimes it is better to do what the so call experts do, rather than do what they tell you.