Let me ask you this first.

Before using a straight razor, did you also get razor burn from using your DE, Fusion or any other shaving tool you might have used?
Do you have acne on your neck?
Do you give your neck enough time to heal from the razor burn between your next shave?
What is your beard type? coarse, normal, barbed wire,...
Are we talking razor burn when shaving WTG only?
Do you go over the same spot over and over again?
I'm sure you prep well with lots of water soaking, good quality soap or cream,...

Many questions but there's many variables involved and we need to slowly eliminate the ones that do not apply.