How is the best way to touch up old furniture or used furniture with stain?

Sometimes it doesn;t take when I apply the stain? Is there a coating that needs sanded or something else I am doing wrong?

If you want to stain any piece of wood, you need to first sand the old finish off of the wood. Part of the old finish is a coating that makes it moisture resistant--it is this finish that is not allowing the stain to take. Also, you need to go down to bare unstained wood, so that the new stain will take evenly over the whole of the piece.

Staining requires that you strip off the old finish before adding a new one. Chemical stripper is the best; I use "Superstripper" and acetone and 0 grade steel wool to clean up the residue.
Only sand if absolutely necessary (sanding can easily ruin thin veneer.)

I am not 100% sure about this but I think you may need to strip off anything that is on the furniture before you put a new stain or color on it. For furniture with stain on it already, perhaps a type of primer or possibly sand it as you said.

wood stains will go away if you put on wood oil, keep on doing it and let it soak up the oil, just remember stains take time to go away if you get other advice keep on trying it and see if you can see a difference in a few weeks. that's if you don't want to hurt the wood by sanding it.

i am a wood furniture builder and it sounds to me like there is some kind of laquer or something that is over the old stain. if there is, you need to remove that first. you can stain over the old stain but if you are going to then you are going to want to restain the whole thing or else you are going to have an uneven look. also if you do stain over the old stain the furniture is going to get darker every coat you put on. if you want the same look you have now then you will need to sand the piece down and start from there by staining it again.