be able to侧重于能力上的“能或不能”，fail to侧重于“能为之而不为的行为”，例如“未能按照……的要求”，这就是一种“能为之而不为的行为”

1例句一

虽经我方多次书面要求，乙方一直未能按照国家外汇管理局、税务局等部门的要求，提供批准退汇所需的相关资料。

In spite of our repeated written requests, Party B has failed to provide relevant materials as required for reexchange approval in accordance with the requirements of the State Administration of Foreign Exchange, the tax bureau and other authorities.

2例句二

Farmers may not be able to prevent natural disasters, but they can at least try their best to reduce losses.

农民不能阻止自然灾害的发生，但是其至少可以尽力减少损失。