Putting the preferred scratching object so near the wrong one as to make the wrong one unusable has worked for me. Sometimes this has entailed temporarily attaching the preferred object to the sofa. Usually I show the cat how to use it. Then I've moved the preferred object to the place I want, meanwhile wiping down the couch or whatever with citrus scent, which most cats don't like.
Some ideas:
Is the scratching post a store bought one with carpet on the outside? Some cats don't like the carpet. They like the woven jute backing better.
Some cats like to scratch standing up, some like horizontal surfaces. It helps to observe which the cat likes best. Some like both.
The absolutely best cat scratcher I have ever found is the end of a cedar fence rail. About a foot and a bit long. The wood is soft and they can really get their little hooks in and it doesn't catch like loop carpet will. I appreciate people who call before visiting so I can vacuum up the cedar sawdust that's always on my rug before they get here.
Our cats also like a small roll of carpet that is always lying on my rug. It flops around and I don't know why they like it better than carpet fixed to a post but they do. It's very low pile, and actually, there's very little pile left now but they still use to scratch and play on.