ROBERT, La. — After failing again to stem the flow of oil into the Gulf of Mexico, BP scrambled Sunday to make some progress in ending the spill that the president's top energy adviser said was the biggest environmental disaster the U.S. has ever faced. Six weeks after the spill, oil giant BP PLC said that its latest plan to cap the well wouldn't capture all the crude fouling the Gulf. And the relief wells currently being drilled — which are supposed to be a better long-term solution — won't be done for at least two months. "Well, the relief...