савол FSX Boeing 747-400 Aerolineas Argentinas Speedbrake issue

Hi Guys, great site! I have recently downloaded the FSX B747-400 Aerolinias Argentinas package. All is great, except when i deploy the speedbrake to slow down on the descent, it does not slow the aircraft down. It does deploy fully, but seems to have no effect on the speed. Also, minimum idle is at around 31% so finding the descent speed quite tricky to control. Any thoughts or help as to how to overcome these issues would be greatly apreciated.