"...inaccurate Navy-supplied wire dynamics model..."
The day of the unmanned aircraft. By Dave Majumdar on May 15, 2013
http://www.flightglobal.com/blogs/the-dewline/2013/05/the-day-of-the-unmanned-aircra.html
"...However, the X-47B did not carry out an arrested landing upon returning to Pax River. That could be because the unmanned jet was having difficulty making even that first trap it did the week before where the Navy showed off a video of the aircraft snagging a wire. Sources told the DEW Line, at the time of the earlier trap, the aircraft now had a 10 percent field boarding rate... So hopefully, this isn't an indication of a major problem. The X-47B guys have had to redesign their tail hook a number of times now due to the same inaccurate Navy-supplied wire dynamics model that was partly responsible for the F-35C's woes...."