I think the other teams have just gotten much more into the mix. The Saubers are knocking on the Q3 door, so are Williams, the Force Indias, and MASSA too! Button was only 3 tenths down on Hamilton in Monaco. That's probably the normal gap between the two but nowadays every last hundredth counts, and you gotta be adaptable to these ever changing Pirelli tyres. We have seen so many times where Button loved the Balance in FP1, hates it in FP2, Loves it in FP3, hates in Q3, the race etc. He is like a lilly, too easily affected by outside changes.
In regard to the New nose, it might be tempted to blame it, but the higher nose is supposed to be favourable to Jenson since it's supposed to move the centre of pressure rearward, more to his style. However, maybe the Pirellis have just thrown any aerobalance tuning out the window.