NotSoSiniSter

Would you happen to know why some people seem to pick up new accents very rapidly, while other struggle tremendously? I have a few friends that have perfect british/australian/russian accents...and I struggle barely speak english to begin with. :D

Do you have any tips to reduce mild mumbling / stuttering? I do both and in varying amounts and it can be quite frustrating / embarrassing in certain settings. Can speech coaches help a lot with this?