Thralls I place at any bench do not begin working or enter the worker slot. They just go straight to the benches inventory. Similar to what was happening with items at the dismantling bench in another separate bug report.
You have to manually move them into the worker slot.
If ever there was an appropriate time for “noob” “newb” or whatever variation. That time is now.
It will get patched. Rough life having to press a few extra clicks 
Lol 1st world problems
But I think its a problem with the jorney steps. Because i have one now of putting a Thrall in a Bench and It doesnt advance