I used to be pretty flexible. Now, not so much. I notice problems straightening my knees in a lot of exercises - crunches with raised legs comes to mind, as well as several yoga and Pilates moves I can't remember the names of. Stretching toward my toes inevitably causes my knees to bend slightly, even if I'm trying to just hang rather than achieve a deep stretch.
I might have mild APT, but I think it's just that my whole core is weak, so posture is slightly affected. It could also just be that I have a deep, round chest and upper back that causes the slight gap between my back and the floor when I lie flat-not sure, really. Also not sure if that would have anything to do with this, but throwing it out there in case it does. I tried the Thomas test last night and had no problem at all -very easy - so I guess hip flexors are fine based on that.
I'm guessing hamstrings might be tight, or is it just the generally weak core muscles causing the knee-straightening problem? I've found lightly supporting the backs of my thighs with my hands is the only way to keep my legs from drooping out of fully extended positions into bent knee positions.