It also doesn't help that your company are going through Union negotiations at the moment Arcal76.
I know for a fact that many of your people are trying to make a point to management by doing absolutely everything as slowly as they can to try and highlight manpower issues and parts issues.
That does nothing to help your availability rate.
Due to your lack of AME's, it takes your guys 3 weeks to do a 300hour inspection. That inspection should be done in 7 days at the most with a good crew working on it.
Try not to blame the machine for shortcomings within your organisation. Many within your company still think they are operating a 76A with no autopilot and very basic systems instead of a complex nearly 7 tonne aircraft with many advanced systems.
The whole debacle with the medical interior hasn't helped things and finally that has been modified as an interim measure, until a new interior is designed and installed.
Until some major changes are made not much is giong to change. Perhaps modelling your operation on other EMS organisations that are using the 139 would improve availability and reliability.
As Geoffers remarked, an external audit (perhaps by another EMS company using the 139) could well highlight where improvements could be made.