Games : How to fix the "Not enough goods to sell" problem

I've created a dozen cities in Skylines so far, but in my latest city, I've encountered a new problem. A lot of my Commercial buildings report the problem "Not enough goods to sell".

From my understanding, it seems like the Goods in these Commercial buildings are selling out too quickly, and not being replenished fast enough.

Here are the ways in which a Commercial building can replenish goods, and my response to each one, in my particular game:

• Trucks. They bring in goods, via the international highway. My traffic is really good, so this shouldn't be a problem.


• Trains. I've got one train station, since the railway is pretty far out from my city, so I had to extend my city just to reach this one station.


• Ships. I've got four cargo docks, and it still doesn't seem to be enough. Most ships coming in are 100% full. Although I've started to notice a few are less than 50% full, which I thought indicated that my city had more supplies than necessary.

Is there anything else I'm missing? I've also got no Policies enabled regarding Commercial zones.