Various considerations for the unwilling designated Dwarven Trader:
1. Check if the dwarf is actually assigned as a trade (assesible through the nobles page, default "n").
2. Check if merchants are actually trading at the trade depot. This can be found out by clicking on the trade depot.
3. Check if trade depot is accepting the designated broker or anyone as the trader. This can be found out by clicking on the trade depot.
4. Check to see if the trade depot is forbidden or materials to create the trade depot (e.g. obsidian blocks) is forbidden. Note: Usually, a message will indicate if a structure is forbidden or not.
5. Check if the trade depot is accessible: Make sure stairs, ramps in the process of being removed, doors are accesible, etc. Also for caravans, to check if they can access them, press default key Ctrl D (I think).
6. Make sure designated trade dwarf has his/her other tasks disabled (such as stockpiling items, stone, etc.)
7. Make sure that not too many items are being hauled to/from the trade port, as the trader might assist in this endevour.
8. If trade depot is located outside, make sure all dwarves are allowed to go outside.
9. Make sure no hostile enemies or creatures are near depot.
10. Make sure you only have 1 accesible trade. More than 1 trade depot sometimes get a little buggy.
11. Make sure you actually HAVE A TRADE DEPOT!!!