There's generally two types of food waste: (almost) at the point of consumption and before that. In richer countries large amounts of losses happen at the point of consumption or just before that, for example consumers, supermarkets and restaurants throwing excess (but otherwise good) food away. In poorer countries most loss occurs in the supply chain (transport, storage, harvesting, etc.).
These two types require different approaches:
Rich countries need to help consumers tackle food waste. That's hard, because there is little incentive for doing so. The consumer doesn't really benefit from wasting less and companies in the supply line also don't really benefit from saving after they've made their money. One way to change that would be to create some sort of incentive, but how do you do that on a large scale?
Poorer countries need knowledge and technical solutions to reduce losses in the supply chain. Here the problem is quite obvious, it costs a lot of money to improve the supply chain, which is hard to do for poor countries. Rich countries can and do help with this, but there's only so much they're willing to do. In addition, there are many foreign aid things that need money, so whatever money is spent on foreign aid gets divided over different causes.
This Guardian article is interesting and should support some of the claims in my answer.