Even so, taking Ramadi and holding it are two different things. Evidence from previous battles suggests that ISIS doesn't do defense as well as offense, and it is still vastly outnumbered by Iraqi forces. But the longer ISIS fighters are entrenched anywhere, the more difficult they are to expel, and the Iraqi Security Forces clearly aren't capable of the task alone.
The ISIS is having only around 50,000 to 100,000 active fighters, which is just about a tenth of the combined strength of the Iraqi Army, Shiite militias and the Peshmerga. Most of these fighters are used for offensive operations, with very few available for holding the territory. When their opponents launch offensives against them, the ISIS conducts tactical withdrawals, in order to minimize the casualties. But as soon as the offensive operations cease, the terrorists move back to reclaim the lost areas.