Question: The Buddha-to-be, when still a Bodhisattva, once gave up his body to feed a starving tigress and her cubs. Was it out of compassion, or out of the knowledge that he would be reborn? There are many dying of hunger every day. Is the Buddha powerless to help?
Answer: The Bodhisattva sacrificed himself out of both compassion for the tigers, and wisdom that he would be able to continue helping more beings when reborn, with even more merits and experience.
Much as the Buddhas (and Bodhisattvas) wish to help all instantly, for one to be helped, there must be adequate karmic affinity and positive karma. For example, see this story for a case of someone whom the Buddha could not directly help: http://thedailyenlightenment.
It is not that the Buddhas (and Bodhisattvas) lack compassion and wisdom to help many who are suffering, but that many lack the ability (e.g. have little karmic affinity and much karmic obstacles) and willingness (e.g. have great stubbornness) to be helped. In the mean time, they are always doing their best to create affinity and nurture creation of positive karma in all. On our part, we can help them do the same.
Among many others, there is an international Buddhist relief organisation that works in some 47 countries that offers unconditional help to the needy: http://en.wikipedia.org/wiki/