Workers had to lift and move aside the the massive box after trying to lower it over the leak.
"The build-up of the crystals in the box made it too buoyant and clogged it up," Doug Suttles, BP's chief operating officer, said.
"I wouldn't say it's failed yet - what I would say is what we attempted to do last night didn't work," Suttles said.
He said BP had anticipated encountering difficulties with the box, but had not expected them to be as significant as a problem.
The containment box had been considered the best hope of stemming the flow in the short term.
Teams were evaluating whether the issue could be overcome by providing heat, methanol or other methods.
The dome had been expected to be operational on Monday and to collect about 85 per cent of the leaking crude by funnelling it up to a barge on the surface.
About 400 metres away is the wreckage of the drilling rig Deepwater Horizon, which BP was leasing when it exploded on April 20, blowing open the well and triggering a major environmental crisis.
Eleven workers were killed in the accident at the platform, which sank two days later.
Methane gas factor
An estimated 800,000 litres of oil have been spewing daily ever since, in the biggest oil spill in the US since the Exxon Valdez disaster in Alaska in 1989.
The blowout was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding, according to interviews with rig workers conducted during BP's internal investigation.
While the precise cause is still under investigation, the sequence of events described in the interviews provides the most detailed account of the blast that has poured more than 11 million litres of crude into the Gulf.
According to one interview transcript, a gas cloud covered the rig, causing giant engines on the drill floor to run too fast and explode.
The engines blew off the rig and set "everything on fire", the account said.
John Curry, a BP spokesman, would not comment on Friday night on whether methane gas or the series of events described in the internal documents caused the accident.
"Clearly, what happened on the Deepwater Horizon was a tragic accident. We anticipate all the facts will come out in a full investigation," he said.
Among the options being considered was to plug the leak by injecting ground-up material in a "junk shot".
"It has certain issues and challenges and risks with it, and that's why we haven't actually progressed up to this point," Suttles, the BP executive, said.
"But we will look and continue to see whether that's a viable option.
"It's all to do with we're working in 5,000 feet of water in a very difficult, challenging environment."
Public concern about the oil spill has been focused on the potential environmental and economic damage.
Workers have also sprayed dispersants over the slick to break it up and deployed hundreds of thousands of feet of boom to contain the spreading oil.
But environmentalists have warned that dispersants like Corexit were also dangerous to sea life.
"Those products don't make the oil go away," Joe Griffitt, a marine biologist at Gulf Coast Research Laboratory, said.
"It just falls to the sea bottom. That's where you'll find the sediments and the larvae. So the toxic effect is double."