Hello Oscar,
We have a similar, but not exact setup and we have the same problem you have. Have you found a solution yet?
Our Resources (Work Centers) have two capacities - Processing Unit (machine) and Labor. The scheduling tab of the Resources says that processing unit (machine) drives scheduling (the Labor does not drive scheduling).
In your work centers, which is the capacity that drives Scheduling - your machines or labor?
Do you use Pooled Labor in your Work Centers? How many Labor Capacities exist in each work center - only 1 or more than 1?
Please answer these questions and I might be able to suggest additional things to try as I troubleshoot my own problem....