Here's a photo (at left) of a graft that didn't callous as well as the one pictured above. Even though the graft was successful, the stock tree didn't provide enough energy to force the rapid expansion of the scion. Without fast scion growth, the quick covering over of the wound made by grafting is impossible. Grafts made late in the grafting season tend to to grow slower (like the one in the photo) than grafts made as soon as the bark slips in the spring.
When uncovering a graft, I often find a colony of ants tending their eggs in the warm, moist environment provided by that aluminum foil and plastic bag (photo at left). Ants usually don't interfere with callous formation, but they can enhance the decay of the stock stump. Once exposed to the sun, the ants will disappear.
A technique for limiting ant nesting is to remove all wraps in August (in the year of grafting) and painting the graft union white. White latex house paint will protect the graft union from sun scald while eliminating the protected environment ants need for nesting.
As always, make sure to support all bark grafts with a sturdy stake until the grafting wound is completely covered by new wood growth.