Here's the most recent example I can offer and I'll keep the details out of it in order to not out anyone, but I can assure you all it's 100 % true.
A well respected sell side analyst publishes a book on how to achieve success in the markets using mostly classical TA. I haven't read the book, but I've seen some of his free analysis over the years and it's pretty much common TA where he's great at explaining what price DID. MAs, trend lines, S/R, etc.
The same analyst leaves his analyst job to start his own fund. The result for 2021? A net loss! I've read his market updates and there's always some nice excuse for why each month pretty much was a net loss.
To be honest, I'm sure he's doing WAY better than most published authors on the markets.
Another guy I followed in the past (but never paid up for his premium content) I also discovered were doing pretty bad in the markets. Considering how he's selling expensive courses I suppose it's a reason he's doing that and not cleaning up in the market place.
Did anyone see Jim Simons trying to teach people to trade?