In my opinion retention is definitely a challenge. How you identify, analyze, approach and find solution may be a difficulty.
As a HR person if you make sure that the employees are satisfied, if you provide them with appropriate monetary as well as non monetary benefits, understand their need of growth, identify their capabilities and enhance and influence them, guide them to achieve then the employee retention for the most part is taken care of.
All in all I think a HR Manager should have a rapport, understanding with the employees.