There are several problems for developing countries:
1) Corruption. This is clear to most people. There are many kinds of corruption done both through illegal and legal means. This is draining a lot of resources.
2) Government inefficiency. Massive issue, often underestimated. In most countries despite the number of government employees, most services are slow, bad or both. A lot of government jobs are life jobs, meaning that the worst people in existence work there and don't get fired.
3) Debt. This one is unclear to most people. Most countries are suffering massive budget losses to interest payments and debt repayment. Instead of the people's taxes being used to improve things, a big portion is being given to rich institutions and money lenders. Debt is good for emergencies or large projects that will generate more revenue than interest and debt payments combined. This is very rarely the case. The rich have managed to enslave most countries this way.

Major improvements on any of these 3 will make a country advance rapidly, and improving all 3 will give you guaranteed prosperity. Do not get debt.