Whatever the reasons, in laymans terms "insufficient altitude to complete a loop" has been seen so many times on so many types.
Is it time to re-consider whether this move (and similar ones) are inherently risky, perhaps a bit too risky?
What are the actual margins here? Even if you make your gate at the top - what margins exist for a technical issue or even just a control error in the 3rd quarter of the move?
It seems the speeds and energies at play here leave very very little room for error when performed close to the ground. Perhaps it is these moves and their inherent danger that need to be under the microscope, rather than the type or pilot experience.
The reds do it regularly of course without it seems difficulty, however I recall the thunderbirds had a terrible multi fatal accident from a loop. From the wiki it seems this was a technical issue (rear stab jammed) though I find it slightly odd all the video evidence was destroyed - a different time/culture perhaps.
https://en.wikipedia.org/wiki/1982_T..._Diamond_Crash