When a high ranked piglet grows into a low ranked pig,
it is when you could not meet the pig's breeding requirements.
As requirements,
1)Give their favorite food to pigs who have unbalanced diet.
2)Make them pasture to the pigs that require "pasturing"
3)Let them eat more than the required number.
4)Let them eat and let them have a certain amount of weight
Those shown above are the 4 requirements although some may vary by type.
Your piglet grew into a lowly ranked pig maybe because some of the requirements weren't completed.
It will be more difficult to grow highly ranked piglets compared to medium and low ranked piglets.