fishfinder problems

[hobb3s93]

i went out on sunday at cherry beach for the baytubers tourney , and the thing is, my ff was reading the bottom perfectly but i wasnt reading any fish . i caught at least 15 croaker in the shallows and when i was pedaling around in the outback out to the islands for some bass, i saw baitfish boils once in a while when there was a sea lion , but i never picked up on anything above the bottom. im wondering if its the transducer, or just the setting i have it on or what. i have a humminbird pirana max240. my bro was out at the same time with all the same gear, (different yak though) and he said he saw fish once in a while. can anyone help?