As a general rule, B&W can take quite a lot of contrast and usually benefits from generous amounts. However, with B&W it's all about greyscale, and if the brightness levels are similar in the subject and the background, contrast will have a limited effect on the subject's prominence. One of the reasons why some of the other edits are working better is because they don't depend solely on brightness. The difference in colour between the branches and the background can be used to differentiate between the two.

If your B&W converter has the ability to brighten or darken the different colours after conversion you could use that to add contrast between the branches and the background. Beyond that you're limited to using differences in texture and/or making selections.

I think you have touched on the most difficult task in editing this image Claude. The flat lighting is the culprit here. In the grayscale version, the lack of contrast and the limited amount of data in the .jpg version also make it more difficult to pull the tree out of the background. My suggestion would be to increase the contrast, crop out the sky and some of the rocks in the bottom of the shot to make the tree more the center of attention, and then use a vignette to further bring out the tree as the focal point. Good luck and keep posting.