I don't really know for certain. It may be the age of the letter itself. It's a bit dated. The painful part is trying to get another one to try to replace it. So far they are the only school that complained about it, though. I have a good physician letter and a great letter from my boss. I also have a letter from my local charity where I do some volunteering.
They told me to go back to that prof and have him write another letter based on the specific classes I took with him. (Maybe it's possible that it's too vague?) I have a hard time believing it's vague, however, because he wrote a full page review of my work ethic. The professor hasn't taught me since his first letter years ago, so it's going to be impossible for me to obtain another one from him. I'm going to try to push through, unless I can luck out and get an online prof to cut me a break.