Oil Spill is the Top Kill. The reason for the long wait is the physics has involved in Top Kill BP is to attempting the pump much drilling mud into the blowout preventing that it actually reverses the flow of the materials, with pushing the oil back down to the well. To be successful their making they needs to push the oil three miles backward into its subterranean reservoir before the well can be capped with cement.

But the BP officials said:

Wednesday night that the effort was so far progressing according to plan, pumping drilling mud into the blowout preventer at the expected rate – roughly 40 to 50 barrels per minute.

Top Kill was not attempted during the first month of the spill because the company needed to know if crucial parts of the blowout preventer were still operational. In addition, scientists could not devise a plausible solution before they had collected data about the leak.

Both of these processes took more than a month because of the difficulties of operating at sea depths of 5,000 feet. BP would continue to try to stop the leak if "top kill" didn't work, perhaps trying to put a second blowout preventer on the well.