NASA has put together an informative page on the challenges of capturing imagery of the Gulf of Mexico oil slick and why new images aren’t posted each day. Terra and Aqua satellites are the only two NASA satellites that can provide pictures in near real-time.
But unlike weather satellites that look at the same region on Earth 24 hours a day, Terra and Aqua generally only get one daylight view each of the Gulf of Mexico. Terra’s view is shortly before noon each day, and Aqua’s is shortly after. The view may be cloudy or hazy, and if the oil slick location is near the edge of the image, it will be blurry.
A combination of factors: cloud cover, sunglint, and the calmness of the ocean affect the ability of either satellite to capture usable imagery of the oil slick.