OK, several possibilities.
Crown of the road.
Check the inside of the shoe for a seam.
Almost everyone's feet are different sizes though not enough to be a problem. You may have enough of a size difference to make shoe fit a problem.
Your left foot pronates more, check the sole wear pattern.
Do you wear socks? There are double layer socks for the blister-prone.
Don't wear cotton socks.
"To give anything less than your best is to sacrifice the Gift."