The reason ISIS is being successful is that they have a singular purpose and identity. They will do what it takes to achieve those goals.
The west is often fraught with second thoughts and varying intent, therefore we wage limited war to obtain limited results-expecting that our enemies will have the same mentality.
When “Allah” is your CIC, you have unity and all actions work towards the singular goal.
When political manipulation is your guiding principle (which changes often), you have confusion and wasted effort.
Another factor: When Islam is in a rampant period and is rolling forward in conquest, they freely grant admission to those who immediately swear loyalty to the caliph.
This means that when “moderate sunni muslims” in the Iraqi army, for example, hear that ISIS is coming, they know two things: If they resist and ISIS wins, they will be slaughtered without mercy. But if they switch sides, they may be able to join the winning team.
So anytime a “moderate muslim” military is attacked by an army of the caliphate, half of the “moderate muslims” will shoot their fellow soldiers and switch sides, or will just hide until the fighting has passed.
Makes it very tough for the ones who really want to fight ISIS, when they must worry about being shot in the back by a turncoat from moment to moment.