Hi,
did you already posted a similar question? I remember reading about a similar problem...
Well, the fact is that it's difficult to give advice without knowing anything about you. Can you make a small audio file where you pronounce the words you have trouble with? And maybe tell us more about the way you pronounce them and what are the specific sounds you really can't reproduce?
I suggest you record your accent, put it on the net and then give us a link here
![Smile [:)]](/emoticons/emotion-1.gif)