for me when this happens, i go to build > move then pick up and place the objects that they work with. if that fails check rooms and set to all. then go back to what you had.

Ran into this after placing some objects down in a tight room, removed some previous placement seemed to have helped unblocking their path, no other clue was provided.

same here and I've made their working area bigger than normal to try combat it but still happens.

having same problem they wont off load the truck nothing in the way

This issue cropped up again for me, just a move update on cutting tables and storage shelves plus auto-boxer resolved it. Didn't have to move their positions just enable move and reapply, was weird.

Not saying it is the answer but thought my game was dead for a moment.