I think the above volcanic ash option is the best. It's a large island, they aren't sure exactly where everyone is, and there is ash everywhere.
As another option, try politics. Perhaps to get a rescue operation flown in they would have to fly through the no-fly zone of a country, and they can't get permission to pass through it? Perhaps the country that has the volcano eruption explicitly refuses help for political reasons. If the country is developing it may not have the means, or suicidal tendencies, to shoot down a humanitarian rescue chopper. However, if they are developing due to presence of oil or other natural resource they MAY be able to threaten to cut off resources to first world countries that don't respect them.
If you claim that only the few dozen folks are in danger, perhaps their on an island that is mostly uninhabited, then it becomes a matter of a few lives vs major political fights. Perhaps the country with the volcano claims that they can rescue the people in trouble and the government needs to 'prove' they don't need help from the big first world countries, but the local government actually lacks the resources to mount a rescue. Meanwhile their stubborn insistence on proving they don't need help, and threats to cut of oil and otherwise punish any country that does provide aid prevents or delays first world nations from helping.
Politically the first world nations would say they are respecting the wishes of a sovereign nation, not giving in to demands to keep the oil flow. They may be taking other actions, like organizing a rescue that works with the local nation's government in a way that lets that nation save face.
To make the idea work well I would imply that the US and first world countries don't think that those trapped are in immediate danger, and thus think they can afford to take some time in playing the politics game. Maybe they underestimated the rate at which lava is flowing and misjudged how close the trapped persons are from the actual lava flow? Perhaps the local government is intentionally feeding mis-information as part of their saving face (yeah we haven't rescued anyone, but not because were incompetent, they just aren't in danger so we don't need to do anything crazy). Between the misinformation, and the ash blocking out satellites (plus, we wouldn't have spy satellites over unimportant regions of the world, so we don't have nearly the bird's eye view you may assume) the first world countries simply don't get how dangerous it is for your survivors.