I had this problem as well for awhile. The cannon was exploding right agains the ground or castle edge, yet everyone else around was shooting off cannon balls left and right and getting the skill point in less then a minute.
I didn’t realize you have to hold the 2/fire button down and a power meter appears a few moments later. Again, there is a small gap of time between pressing and the power meter appearing. After that, the challenge went fine, things were being blown up, and the red thumbs down flags on the castle edge in front of the cannons turned into green thumbs up and a skill point was had.
Hope this helps.
—Gesten Nyotas.2193 of Yak’s Bend