FFK, realised what the problem (probably) is.
The auto-throttle is ONLY a throttle. It cannot hold a given speed regardless of the attitude of the aeroplane. If, for example, you put the aircraft into a dive - or tell the autopilot to lose height quickly, for that matter - it is likely to fly a lot faster than your speed setting on the way down. All the auto-throttle can do about that is shut off the power - once that is at 'idle', it has done all it can! When you level off, of course, the autopilot can do its job again and hold a given speed.
In the specific case of a landing approach, if you get the trim and attitude right - a descent rate of around 600 feet per minute with around 30 degrees of flap and gear down - you should find that the auto-throttle will hold the desired speed. But not if you dive steeply. Hope this helps.
"Once you have flown, you will walk the earth with your eyes turned skywards.." - Leonardo da Vinci