I've also had problem with my neck - for a long time, I just couldn't get it as smooth as the rest of my face.
I've found that not only do I have to pull the skin really tight, but also that a slightly heavier stroke is sometimes needed on the neck. The skin there being a bit more used to irritation, it doesn't give razor burn or anything like that, but it does take off the hair much more efficiently.
Now if I could only get right under my lip and my moustache....