Because of the ways your loved ones addiction is effecting their brain they are totally focused on their drug of choice and they do not think in the same way they did before they became addicted or that you do now.
It is important to learn how to be very consistent in your communication with the addicted person. Avoid arguments and manipulation like the plague. It's fine to be honest with your loved one about their need for treatment or help of some kind. Trying to manipulate them with guilt, anger, threats or money doesn't work and is destructive to the relationship.
When we are trying to fix them or rescue them or manipulate them anxiety becomes the major force in driving the relationship with the addicted person and all the interactions with the addicted person become about trying to gain control. The relationship becomes a power struggle marked by confrontations or fearful avoidance.
The main problem is that family members know their loved one is addicted to drugs but they don't accept this fact and they try to fight it.
By accepting your loved ones addiction doesn't mean that you like it, or support it, or don't work toward alternatives, but if you want to have a relationship with your loved one that offers some hope you need to accept the reality of the situation. When you wind up in power struggles the main issue actually gets shoved to the background and the addicted person can focus on their resentment toward you and you can struggle with trying to manage their addiction. It is quite a dance and needs at least two or more people to coordinate the steps.
If you want to have some control in the relationship you need to give your loved one a large field in which to roam.
The more effort you put into trying to control the details of their life the more they will fight against you. You will become increasingly frustrated and feel increasingly powerless.
Addicted people ask for help when they experience serious enough consequences because of their addiction that break through their denial. These are generally the natural consequences of their addict lifestyle. Sometimes this change comes on gradually and sometimes if is sudden.