As a learning & Development expert , what's the challenge that you face and how to overcome?

I'm not personally an expert in this field. However, I do believe that one of the biggest challenges you might face in this area is motivation people to actually learn in order for them to become better at whatever it is they need to learn. It is very well known that people don't react well to change, but if they are convinced that the development will be positive and will have positive measurable results, that might make them more interested in learning. 

I'm not an expert but a goal is the first step. After that, easiness , clearness & motivation .

The biggest challenge is to facilitate learning for people who don't want to learn and are put in the classroom by their manager. An engaging model of learning with encouraged audience participation often helps tackle that scenario!

Resistant to changes and unnecessary desires are the most severe hurdles in the learning process, but if you are willing to sacrifice some thing in exchange of another then these hurdles would easily be eliminated.