Mclaren seem to think they've solved the issue with JB -
http://www.autosport.com/news/report.php/id/100515
I'm going with one of two possibilities, they let Button do his setup in practice, then just stick Hamilton's setup on Button's car and don't tell him or two, they just make Hamilton a bit slower
Serious answer comes across as something quite simple, there is no problem with the car, Hamilton's setup works, Button's doesn't.... Button has no clue how to set up a car for that last half second of speed.... didn't matter with Brawn when he had such a humoungous lead to start with, didn't matter in the past two years when there was no one really close to Mclaren(behind them that is) with the fast cars basically ahead of them, this season that same time is costing him real places.
Fact is I've never once watched Button and thought, great drive, aggressive race, he'd have beaten anyone, never once.
Even the much talked about last year's Canada, Hamilton was faster, spun, and was catching Button hand over fist when Hamilton hit him in the back/Button put him in the wall. Before a safety car, bad weather and a fluky pitstop he was running last IIRC. He then passed half the field in the pits and did rather what Hamilton did at the end of Canada, caught cars with rubbish tyres ridiculously easily. That isn't good driving, 5 seconds a lap faster isn't great driving.