The AI is apparently not able to use the traders beneficially and flexibly. I have often observed that although enough traders are built, they often remain mostly (sometimes all) idle at the trading post. This is despite the fact that there are enough trading posts within easy reach.
Apparently the traders are assigned a fixed trading post at spawn. But if this post is not reachable because of walls or enemy troops, the traders remain inactive.
A dynamic assignment must be programmed here.
Thank you for reporting @BilboB1887! The team will investigate.