Argentina defaulted on its sovereign debt in 2002. It has reached an agreement with most but not all of its creditors. Wikipedia has a pretty good summary. The agreements included a 70% hair cut but also a RUFO or "Rights upon future offers" clause stating that if in the future agreement is reached with the holdouts with better terms (lower loss percentage) those better terms automatically apply to the creditors signing the current agreements. (This is similar to a most preferred customer clause.) Perhaps the Argentine government used this as a negotiation tactic as it makes it impossible to offer better terms to the holdout creditors.
Unfortunately the holdouts bought a lot of CDS insurance and now stand to win more from a default then they do from a 100% repayment. Lawsuits lasted for years. Argentina made payments to the creditors it agreed with but made no payments to the holdouts. In 2014 (maybe 2015) a court blocked payments to the non-holdout creditors because you cannot differentiate between identical bonds in case of default. This is plunging Argentina into default again which is bad for the country and the creditors with whom it already came to an agreement. (There is another court hearing scheduled for April 2016 and the US government is advocating in favor of Argentina.)
Why haven't the non-holdout creditors and Argentina signed a new agreement in 2014 without the RUFO clause but identical in all other ways? Seems like both of them would have benefited from it, just as they have by coming to an agreement in the first place. (I am not necessarily looking for the 'exact' or 'real' reasons. Any plausible reasons that make economic sense are fine.)