On a slightly more academic level than /u/sertorius42 's pithier summary:

Global strategy is built around the 'Balance of Power'. Right now, the world fundamentally disagrees on what the balance of power should be.

Prior to 2008, the US was considered the last Superpower and given responsibility to police the world. They attempted to create democracy in the middle east, but failed spectacularly due to poor planning. At the same time, the other great powers dispute America's role as world police. They sometimes offer to play a role in regional events (such as Iran helping with ISIS, though Iran isn't a great power and is barely even a power).

What happens is that in every conflict, each country has a slightly different agenda. Syria, as Sertorius said, is an ideal example. The US wants to see the rebels win in Syria to create a more progressive and western society. Turkey wants to see the rebels win, but only after ISIS has cleared out the Kurds - whom they fear will ally with their kurdish minority. Russia wants to see Assad win, because Russia feels that a progressive dictator is better than a progressive democracy and will be more stable.

So, everyone agrees that something should be done in Syria, but we all oppose each others' plans, because each upsets the balance of power. Syria has been a poor ally for Russia for five or six years, and seeing that restored would embolden Russia at a time when the West is trying to contain them. Similarly, losing that mediterranean foothold would weaken Russia's position to defend Crimea. Lastly, Turkey prefers to draw the whole thing out because it minimizes their chance of losing territory at some future date. Unable to proceed without upsetting the balance of power, we delay and hope to find a solution that does not assist or harm any of the major players.