The spokesman (AAIB?) I saw on TV last night said the plane was at 600 foot when it dropped very slightly below glide slope (journalists - note he didn't say that in itself was abnormal) the auto throttle commanded more power as it is designed to do but engines didn't respond, (can't quite recall exact words but something like) pilot then switched to manual and advanced the throttle but the engines still did not respond. Presumably they continued to drop below glide slope (which obviously was abnormal!).