Logic and justifying one's answers

I am grading the exams and I feel like I must not have done a good job of teaching biomagnification this year. Sigh. You fix some problem they had the previous year and then something that was fine the year before becomes a source of confusion. Rats. The general story is one of not justifying one's answer with logic. But hey, according to Pam they mostly still can't do that by 4th year. And I noticed last year that even if they got good at working logically through ecology and genetics problems that ability doesn't transfer very well. When I gave them a problem on the final that would require logic and "putting it all together" for evolution very few people managed. Others have said the same....how untransferable these skills at logical problem-solving seem to be. It's a shame because I see it as one of the biggest skills in their university student tool kit.