There are a types of cow horns that will work ,most won't. I must have tried longhorn 5 or 6 times ,it only worked once. The problem seems to be that cow horns grow in a spiral ,and each year as the length of the horn grows ,the slope of the spiral changes too.Making each years growth of horn run at a slightly different angle. So at some point (usually when I boil the horn) it delaminates between the layers.
The one bow where I got longhorn to work ,I cut the horn into 3" pieces ,these were short enough so that the spiraled layers didn't diverge.
There are cow horns that grow straighter, not sure witch breeds.
Try looking for sheep or goat horns ,I've had good luck with both.
Ralph