Some people just aren't built for pull-ups. There was a time when I could run consistent 6 minute miles for hours on end (well, an hour or so), perform 70 pushups or 80 situps on command in under 2 minutes, curl 70lb weights in either hand for many reps, bench a bit over my body weight, scramble through obstacle courses all day long...
But I still had trouble doing 2 pull-ups in a row. Now, I can't do one, though I could still pass an army PT test (with an alternate event in place of the run, anyway).