I did Kyokushin karate to Shodan. The board we broke were solid pine boards. They are definitely not "cut". My technique was a little off, and I wrecked my knuckles for months. They ended up permanently moved.
So maybe all karate isn't equal.
So maybe all karate isn't equal.
Take one of your boards and hold it with the grain horizontally instead of vertically. Try not to break your hand.