It's probably the environment that your subject is in - reds, browns and greens convert to a similar shade of grey in B&W. That's why the contrast isn't quite there in your photo.
You could try applying a red or green filter to make your subjects stand out through post processing to bring out the contrast. Better lighting that drapes itself around the posts - the sort you'd get early morning or late afternoon will also help bring out a bit more contrast.
Good luck
