Its never going to be perfect shaves to begin with anyway .. Its all about experience and muscle memory this game.
Rash around the neck is a common problem, may be related to your technique being off, little to much pressure, or just not yet having the full map of your necks growth.
Stick with it, Take your time, don't vary your shave setup yet - narrow your potential problem areas, and dont expect to much to soon.
Give it a month or two and you will be flying.