In its natural environment a cat will scratch at objects to ensure that its claws are kept in good condition and there is a belief that it is also to mark territory in the hope of attracting a mate. However, cats seem to claw objects because of the texture or the scent, perhaps just because they feel right or a possibly a mixture of all three.
It is because of this uncertainty as to why the cat chooses only certain objects to scratch on you may well find difficulty when training your domestic cat to use a cat scratching post. Finding the cat's particular preference to certain objects and surfaces is the key.
Never the less it is certain that a scratching post is a definite and valuable item of equipment.
The trick to getting the cat to use a scratching post is to observe its actions. If it ignores the scratching post and scratches elsewhere at a different location, move the post to the cats chosen location. If this fails you could try adding to or covering the post with the same type of material you have found the cat clawing, carpet or curtaining perhaps. You could also show to the cat how ideal the post is by placing its paws on the post and pulling them downward.
However, there will be in the end no guarantee that your cat will use the post but to avoid damage to your furniture and shredded curtains it does make good sense to persevere with training your cat to use the scratching post