You might be right about him not being a good poly candidate. It happens. But he COULD have passed his name, address, DOB, then showed deceptive on the next one, two, x number of questions that might have been involving Dylan.
I have to say, what if, instead of answering with a direct answer on a poly, Mark used his usual word salad. I don't know how anyone could get an accurate reading off of that.
As an example, the following not supposed to be derogatory, it is just how I hear Mark when he talks:
"Did you have anything to do with Dylan's disappearance?"
"Well, you know, I didn't see him when I came home from Durango, and, you know, he wasn't where I left him on the couch, so I thought he'd gone outside for a while. I didn't think anything of it at the time, so I laid down for a bit, you know, because I do that when I have the opportunity..."
How could you get any kind of reading if the answer was like that? In that case, "failed miserably" would make sense.