It's not particularly surprising that ending poverty isn't that simple. Most developed economies were brought to their current stage through hundreds of years of innovation, investment, protectionism, and use of inexpensive raw materials and labor from the global South (among other things), and the ones that industrialized more quickly (like Singapore) often had unique geographical or political characteristics that aided this. The development of a stable, diverse economy (and correspondingly high standards of living for a population) depends on far more things than a simple infusion of capital.

For that matter, poor African states are usually poor for very different reasons. Congo's poor despite its great natural resource wealth because the Belgians systematically sabotaged its ability to rule itself before independence even arrived, in the early '60s, and then it got stuck with corrupt dictators who robbed it blind for four decades. Rwanda's poor because the genocide destroyed everything it had built, and even the stable, non-corrupt government that's in power today can't overcome such odds in a generation (and has virtually no natural resources to help it along). Botswana's actually not that poor, because it has a competent government that's used its diamond wealth well. Rather like different aid organizations are differently competent, different African governments are better- and worse-positioned to benefit from A) aid or B) participation in the global economy in general.