I too have trouble with shaving my neck.
One of the things that helped me most was finding out which direction to stretch the skin.
Before the shave stretch the neck skin in slightly different angles while feeling the whiskers in an against the grain motion with the fingers of the other hand.
I found that if I stuck my chin up and to the side as far as it would go while streching and feeling I could find a combination that would make the whiskers stand up that are normally laying flat.
The other thing that helped me was to use a very light pressure against the skin. The effectiveness of each pass was greatly improved and the light pressure allows for more passes without razor burn.
The pressure thing reminds me of my stropping technique. Light pressure there makes good things happen while heavy pressure does the opposite.
Anyway, good luck to you, and just keep trying new techniques and your shave will improve for you.