Part of the difficulty with writing out code (on paper or on a whiteboard) for me is that I type much faster than I write. My output buffers get filled very quickly with writing code down by hand and things get all jumbled up since ideas are flowing faster than they can come out. Mediating this process takes practice.

OTOH, I can draw pictures by hand much faster than using any sort of software to do so. (That's primarily because I haven't taken the time to learn packages like visio or dia or whatnot properly.)

Writing on a whiteboard is also a different experience than writing on paper, even if you're by yourself. You need to remember to keep the pen point slightly downward so the ink keeps flowing, and to write bigger than normal so other people can understand you. These sound like a stupid, small things but writing is something that's been drilled into you though rote from such an early age that picking these things up can be tricky, particularly if you do it infrequently. (People who attend lots of meetings are pros at this -- one of the best persons I've seen with a whiteboard is fellow perlmonk clemburg.)