The U.S. is assigning more troops, perhaps as many as several hundred, to help raise the training standards of the 15,000 Afghan Border Police. This force is responsible for guarding 5,400 kilometers of land borders, plus several large international airports.
The biggest problems the Afghan Border Police have is too little equipment and training, and too much corruption. Typically, many commanders don't mind losing police under their command, because then the pay of the missing cops keeps coming, and can be pocketed by their commander. It's an ancient scam. Then there are the bribes. Drug gangs pay well, as do people wanting to drive illegal cargo into, or out of, the country. These bribes are also an old custom, which is difficult to halt, or even regulate.
The border guards devote most of their efforts guarding the hundreds of roads (most of them primitive, being dirt or gravel). It's the roads that are favored by people with cars or trucks, including smugglers, terrorists and the Taliban. All three of these groups have been known to attack border guards (who could not be bribed, or who wanted too much). The intent of these attacks is to intimidate the guards, not just chase them away and drive through (because the damn Americans could later catch up with you using their missile armed UAVs).
Training and equipping the Border Police is hard enough, convincing them to abandon the corrupt practices is far more difficult. Many forms of corruption (payroll padding, bribes from truckers) are seen as a fringe benefit. With some persuasion, you can get most police to try and keep the Taliban and other terrorists from getting past checkpoints. But the drug gangs pay too well, and most border cops cannot resist.