It's not clear to me why it's been relatively difficult to persuade some countries to swap out the enriched uranium fuel. From what I've read, replacing that fuel with the appropriate and available low-enriched fuel is pretty straightforward.
It may be that the countries want to be convinced of the safety and effectiveness of the swap, which is fair enough. It's also possible that they need to do it in a way that responds to their internal politics. Being seen to just knuckle under to the bigger powers can be damaging to some politicians at some times, varying with the country. US commentators and readers frequently ignore other countries' internal politics while tolerating near-insanity in our own.* But that's another post. Just consider that other countries have their own internal issues that someone is likely to make a fuss over, even if we don't think it makes sense.
For Pakistan, there has been a great deal of external commentary on a) its possible inability to keep its nuclear materials safe** and b) the degree of its cooperation with the NATO war in Afghanistan. Add to that a relatively weak government that has had an assassination and major unrest within itself over the past several years, and you have the ingredients for balking on something that might seem straightforward.
More about Pakistan from Pavel Podvig and the New York Times.
*Mark Landler provides a nice example of US-centrism in the lede to his Belarus story:
The Obama administration scored a victory in its drive to curb the worldwide spread of nuclear material with an agreement on Wednesday by the hard-line former Soviet republic, Belarus, to surrender its stockpile of highly enriched uranium by 2012.Oh yes, that's the way to reward cooperation: "Scoring a victory" for the US (USA! USA! USA!) while Belarus "surrenders." How about this?
Belarus joined the nations locking down enriched uranium against terrorists.or, if you need something with a little more SOCK! BAM!,
Belarus dealt a blow to terrorist nuclear dreams by asking Russia to take its outmoded reactor fuel.It's really not necessary to shove America's power down Belarus's throat.
**This article is not much different than articles I've seen over the past year or so. Wikileaks provided very little additional information to it.