Often I think hair on the neck can lay at varying angles. Initially, a beginner may not recognise this and so resort to increasing the pressure to shave the area and this can cause razor burn.

Check the angle your hair grows at; you may need to go without shaving a few days to be able to see this.
You may need a second pass on the neck to get it really smooth due to the hollows, direction of hair (esp if it's growing across your neck). For the second pass weaken the lather with a little water. I think a buffing stroke works very well here; very short, light strokes repeated rapidly.