It's all of the above and then some. Some of the settlements were poorly crafted. There was bad statecraft. We've seen a number of cases--quite a few cases, in fact--in which that has been true.
We've also seen settlements that required large infusions of development assistance, external support, and the like, which did not flow in the way they were promised because new conflicts emerged and international donors' attentions were diverted to other regions and settings. There was also simply bad governance. Spoilers who were enticed either to become part of the political coalition or to form political parties have been thwarted because of corruption on the part of political elites.
There's the problem of state failure. These are often very poor countries in hostile neighbourhoods; sometimes you may have a peace agreement in one country, but there's instability next door, and that tends to spill across the border and undermine the very fragile peace process that was set in motion earlier on.