You don't want it to callous. You want it to throw roots. When it callouses it is healing instead of throwing roots.
Sometimes this means you didn't make your airlayer girdle wide enough (so the bark below and bark above doesn't heal across the wound) and sometimes it means that you didn't remove ALL of the cambium in the airlayer, or perhaps left some of the phloem.
When you girdle the tree you need to remove EVERYTHING down to and including the phloem. The removal of the cambium keeps the tree from healing. The removal of the phloem blocks carbohydrates and photosynthates from flowing down the trunk, and these excess materials gather at the wound site and trigger root formation. The level beneath the phloem (called the xylem) is what transports water and nutrients UP the trunk and is what keeps the part of the tree above the girdle alive. So you want to cut your girdle deep but not TOO deep. Fortunately the xylem is located in the deeper woodier layers of the trunk and it is pretty hard to cut too deep. In almost all cases air-layers fail because you girdled too shallow, or left some cambium or phloem behind.