How unions are generally formed is that the workers at a company take a vote, and if a majority vote for collective bargaining, then all the workers get represented by a union in negotiations with the employer. And many companies negotiate with the union and make an agreement where all employees have to be members of the union and pay union dues as a condition of employment. Some conservative states, however, pass what are known as "right-to-work" laws, which prohibit employers from requiring union membership and dues as a condition of employment.
This creates a problem for unions, because unions spend time and money negotiating a contract for all employees, and employees get the benefit of higher wages and better working conditions regardless of whether they're actually paying dues. So we basically have a collective action problem: employees have no incentive to pay dues, because they can just freeload off of other people paying dues. Thus the rates of union membership in right-to-work states are dramatically lower than in non-right-to-work states.
To the extent that changes in working conditions impact all workers, there's nothing that unions can do about this, but concerning salary and health benefits, why don't unions take advantage of the provision in the National Labor Relations Act that allows them to negotiate members-only contracts? This is where the contract negotiated by the union only applies to dues-paying union members. This way, people won't have an incentive to avoid paying dues, because then they won't benefit from the union contract.
As far as I'm aware, pretty much no unions in right-to-work states negotiate members-only contracts. Is there a reason for this, like legal or logistical obstacles? Would there be disadvantages for unions if they did this?