Why is it that when I link a contact to a Facebook Image it will unlink randomly?

I've noticed on my HTC Sensation, I have all my contacts linked to Facebook but the problem is I imported a lot from my old phone with an old image attached as the contact image. I told the phone to just use their current Facebook image as the contact image. However every now and then with random contacts it will go back to the original image as the contact image and not the current Facebook one. Why does this keep happening?