I tend to do a lot of portraits and I personally use 100 sometimes 200, using the camera outside i use 80 unless it is a very dull day then I up it to 100 or 200, If I go out at night say to a family get together in a club I will go to 400 but no more than that as I feel the images are too grainy and I dont like them that way.
What angle have you got the flash at? have you tried other positions? I usually have mine at 60 if i'm not bouncing it back, mine has a diffuser on it though so it will still throw some light forward
Heres one you might want to try adding a peice of card just to see if improves anything like
this