What difficulties do the companies face to maintain their best employees?

A company will be able to keep their employees when the environment is pleasant and it gives a feeling of being able to influence to own work, feel the flow and is overall positive.
Unlike the common belief, money will not make employees stay.
Instead there are so many ways to take care of employees, I work as a consultant in this field.
Investing in staff is a powerful 'weapon' to gain competitive advantage compared to competitors.

pay them more money

Truely the Human Capital is the most important one, and the intellect associated with employees of an organization has no alternative.
THe problem is a lot of companies do loose their valuable asset due to hasty decisions for example:1) There are some black sheep amongst your employess to be dealt without effecting the goodwill between the organization and the employees so as to retain your best employees.2) keeping in view the current economic conditions your organization has to downsize, which should be done keeping in mind the job security concern of your best employees.3) The compensation and benefits have to be in compliance with the local market standards so as to saisfy the financial needs of your best employees.\ ( Though this is not an exhaustive list but it give us the glimpse of the problems face in employee retention)

Muhammad Ismail Rajpoot I think the growth up and development is one of the most important things should the company look at them for the talent employees.