Another possibility for your convoys turning around and leaving is that they had 'Do Not Unload' checked. They'd simply turn and leave if this was the case. I've done this a number of times in the past. Believe me, once you've done that to a carefully loaded amphibious convoy during an atoll invasion and blown your timing you'll be double careful in the future.
Also remember that troops loaded in combat mode for an amphibious assault do so sub optimally EG: they use more space and so will require more ships. The one little ray of light here is that it is impossible to load strategically packed troops onto an amphibious task force. Do not
ever, ever, ever, ever try to land an invasion force on an island or an atoll without the task force being amphibious. It won't go well for you.
. In fact, any seaborne invasion should be a proper amphibious one. Only use the 'Transport' taskforces for moving troops from one rear base to another.
As for coast watchers and so forth. A lot of the time they come out with rubbish but that doesn't mean they won't pass you a gem of information sometimes. I had one give me accurate information about a Japanese carrier one time. It turned out to be true and it got sunk. Keep an eye for anything that pops us like 'Japanese carrier aircraft spotted' or 'Torpedo bomber spotted'. and take a note of the co-ordinates. It's most likely nothing but it could also be the KB moving towards one of your bases.