Hi friends,
Why is there no stock on some ports

Because it has all been purchased. Ports have limited production of their own. Plantations now produce most but that is not evenly spread out..
However, once a ports stock drops below 10mil crates, it will drop 35k extra crates once per hour.

Tortuga was giving me not much returns then for whole 2days

What a bad move

Its a fact that traders have to monitor their routes much more these days. You could buy up stock when its there then have your ships load from your warehouse when the market is empty. See the options in 'trade route' creation.
Another factor folk can miss is that prices can now create a loss. It used to be there was aleays at least 1gc profit but now that is not always true.

Thanks Danik