If they're not doing the work...
I'm a freelancer and if I don't do the work I'm contracted to do, then I don't get paid. If an employee isn't doing their work, then you should explain to them what the consequences are, like getting fired.
Since you're the manager, you have the responsibility to make sure that the work is getting done. Keep a bit better track of the work done (and how long it takes to be done via waiting for lists) and show them the evidence in a monthly, bi-monthly, 6 month or yearly interview. Show them the evidence of how much was expected of them as output, the tasks that were delegated/assigned to them and using your waiting fors and other information, show them they are not meeting the standards that they agreed to by working in that position.
After that just outline the consequences, according to your business culture. -get fired, demoted, re-assigned, reduction in pay, or whatever works for the situation. Just make sure they understand it's a consequence of their own action (or inaction) by showing them the evidence you have gathered, and not because you "don't like" them.
I'm not saying you should just fire someone because of trivial things. People are not expendable like parts in a machine and should never be treated as such. But if you're following the GTD model then you should be flexible enough to allow for both marginal adjustments and more drastic measures.
While I personally believe that fear is by far the worst motivator of all, some people have been trained (by culture, family, history, church, etc.) to only act when motivated by fear (lose their job). If you're willing to invest the time, money, and effort to train that fear out of them then good for you. But if you're not, then maybe a new employee (who doesn't have this problem) would be a better choice.