Greetings,
Remember, FMS will try to keep your flight path within the optimum path,keeping the optimum speed, this is whyit gives you PREDICTIONS to show you how far of you are going to be speed and alitutde wise, and of course with that prospect you can do what the FMS cannot do.
IDLE factor will not solve your problem, check that all data have been entered (wind and temp are very important) check that the cabin press is within the scheduled descent, and if all are correct then he FMS should be able to fly it.
Now regarding TOO STEEP PATH, it just means that the FMS will not be able to fly the path, in the configuration , conditions a that moment in time (may be strong tail wind...)
BTW what box is it?