I find that I have problems understanding people when:
* they have a non-standard French accent
* they are old men. Children's, teenagers, and women's voices are just so much clearer. I find the voices that are the easiest to understand are those in their late teens or early 20s. They sound like adults but still sound young and clear.
* the recording is of poor quality. A lot of the time I can't understand the people I chat to because our connection isn't that great and there is a lot of background echoing noise.