Not enough thrust?

[escott26]

It seems like even with ATS engaged and the thrust levers fully forward, I am still not producing as much thrust as the magenta marks indicate. At FL400, I continued to slow until I almost stalled because the thrust would not increase with the levers fully forward. Have I done something wrong?

[Graeme_77]

Performance is very limited above FL350. Check the table in the Operations Reference, in the documentation folder to make sure you're not asking too much of the aircraft. Maybe you also picked up some ice on the climb? Remember the wing anti-ice if you need it.

There is an engine core speed limit that comes into play too in certain conditions, that's why the engines are not at the max rating on the gauges, low airspeed is a consideration in this. 

A screenshot of your PFD and the aircraft weight would help.