Strategypage explains the problems:
A more difficult problem was getting Afghan commanders to cut back on putting most of their troops on checkpoints and what amounts to guard duty. Instead the Americans are urging the Afghans to operate like the U.S. forces did since 2001. That means using the intel and (especially in the case of Afghan commanders) knowledge of how the enemy commanders operate to carry out attacks and raids on the enemy as soon as you locate them. That means having most of your forces trained and deployed as combat units ready to go off to raid or ambush the Taliban, especially those preparing to attack something (a town, military base or plant a lot of mines and roadside bombs along a vital road). In other words, attack the enemy before he can attack you.
Losing the initiative is always bad. You don't need to look farther than Iraq in the first half of 2014 when Iraq lost the west and then the northwest to ISIL to see what a checkpoint army's limitations are. We failed to stay after 2011 to train the Iraqis as a mobile force to finish off al Qaeda (and we're still trying to remedy that to liberate Mosul from ISIL control).
You can only spread out your forces if your outposts can hold off attackers long enough for friendly firepower and reinforcements to arrive to inflict heavy losses on the attackers.
Add in mobile operations that seek out the enemy to kill them and disrupt them, and your outposts are more secure.
Do all that and the enemy is more unwilling to attack your outposts. Which means you control the territory and people.
Can we help the Afghan forces do all that in 2016? We are increasing our air power and ability to work directly with Afghan unis on the ground:
Afghan soldiers and police know they are more effective fighters than the tribal warriors, but that their job requires them to expose themselves to danger regularly in order to maintain control of territory. The Taliban are not tied down nearly as much and that makes a big difference in morale.
Afghan military leaders point out that these operations are most successful and less stressful when they have American air support and the U.S. has apparently responded to that by quietly sending more warplanes and helicopter gunships to Afghanistan, along with more ground control teams to work with Afghan ground forces. But the air support is still much less than what it was when there were a lot of U.S. ground troops in Afghanistan.
It was the "good" war, remember. The war that President Obama escalated dramatically. If we aren't trying to win, what was the bloody point of sacrificing all those Americans in those surges?
Indeed, if we just walk away from everything, what was the point of electing President Obama at all?
We may have responsibly ended the war in Afghanistan, but the Afghan government has a way to go before they can declare mission accomplished.