The UN effort to recruit peacekeepers has no shortage of troops offering to do the job. In most Third World armies, troops are paid half, or a third (or less), of what the UN offers. Moreover, the UN pays $50,000 in death benefits if a peacekeeper dies in action (much less if death is accidental, which is much more common). This is a fortune in many poor nations, more money than a working man is likely to see in his lifetime. But the main problem remains poor training and leadership. The UN now knows from past experience that disaster awaits if they employ poorly trained troops. These are usually accompanied by unqualified officers and NCOs. 

The negotiations for who has acceptable troops for peacekeeping, and who does not, are handled, well, diplomatically. One would expect that from the UN, but its also practical. Going public about the shortcomings of a nations armed forces angers not just the nation in question, but also the dozens of other countries who know their troops are equally deficient. In addition to being discreet, the UN will sometimes use its connections to arrange additional training for troops and officers. The United States, and some European countries, often stand willing to provide this service. It depends on the foreign policy needs of the donor country, but it doesnt hurt to do the UN a favor. You can always call it in later.