If there is not an issue of allergies,,, then many times the skins begins to itch due to dryness. Itching is often the skin's way of crying out for hydration. The skin below the jawline(the neck) responds to edges differently & post shave treatment differently than the cheeks & upper lip. Alum is great, I use it after every shave, but it must be dabbed off with a moist towel after a few minutes of application. Then I apply a good witch hazel like Thayers. Witch hazel can & does prevent irritation due to swelling inflammation, remember it is a common ingredient in a lot of hemorrhoid treatment creams/wipes.
Witch hazel & a moisturizing cream after you shave, would be my advice.