Two suspicions, which others have mentioned: 1) the plane needs to be really sharp and 2) it needs to be adjusted for a very thin cut.

My additional comment, which may have been discussed on this board in the past, is that working with a sharp plane is very satisfying. That makes it very easy to overuse it during the smoothing/fairing process. I'd rather plane than sand any day, but I also want to follow the easiest path from raw strips to finished hull, and that may not include a whole lot of planing.

Mike