Help with task scheduling

Our project work policy is set to Fixed Work.  Our resources are assigned to more than one project and as a company with users outside the US, we assign our users to a country group with an assigned calendar for holidays, etc.  In addition, each user updates his/her personal calendar for non-working time (vacation, training, etc).

When we set up tasks, we may set the work hours to 8 but the duration to 3 days to allow for more flexibility.  

We are finding that the enforcement of non-working days is inconsistent and due dates are falling on non-working days.  

I need some help in working around this challenge.