I don't know if it is just me или not, but it mentions he has a dark mark in the book , however, it does not Показать it in the movie... Any information on that?
We don't see all the Death Eaters marks in the movies. It's just implied that, if you're a Death Eater, you've got the Mark. We also, technically, never see Severus' mark in the books. He shows it to Fudge, but he's at an odd angle and Harry can't see it.
It was probably due to time, since it wasn't necessary for everyone's marks to be seen.
There are many things they dont Показать of Snape and others, they also didnt have him get his story told by harry in the end( which pissed me off). etc.