I would hazard to guess that the graft wasn’t held together tightly enough. A simple wrap of Parafilm is rarely sufficient. Stretching a grafting rubber (or Temflex, etc.) over it would likely have provided the necessary tension.
I did the same thing (Parafilm only) the first time that I tried grafting, with similar results. I used grafting rubbers over the Parafilm the next time, and it made all the difference.
From my limited experience, parafilm alone cannot get the graft union to be tight enough for callousing. This is especially true for cleft grafts, where there is tendency for the rootstock to split open. Of course, on apples and pears, this may not manifest as much as they are easy to graft. A layer of rubber tape (I use splicing tape) will improve the success rate here.
What you said. My technique is simply not good enough to get a perfect match, and I need to be able to pull hard to help bring pieces into better contact.
You gotta wrap those cleft grafts in a rubber band or something. Even if they make it thru this year, long term viability is not guaranteed.
Not saying you might not get lucky, but if you want maximum odds of success, do it.
Believe me, I learned the hard way.