Hey, as RYUUJINZERO said you need to make sure you select the group and the correct attack option (if you see the HAND symbol at the bottom then they wont do anything) I turn mine to fire at will. you do this by left bumper and push down or the right thumb stick. however, make sure you turn it off when you approach land (left bumper and down on the d pad) or they will just shoot at everything and waste your cannon balls. you can also go into the settings of each cannon and change the range. I've done this to hit the ships further away. once you get used to selected the correct group you can then try the other options mentioned by RYUUJINZERO. it shows you on the right of the screen the button combination you need. its not as easy as it should be so may take a little practice