WorkFlow can't assign 'Assign To'

I’ve set up this workflow to assign a call to a user when an opportunity stage is changed. See screen shot. This doesn’t work at all when I try to set it to the Assign To of the Opportunity. It works only sporadically when set to Random or Round Robin. Can anyone see what I’m doing wrong, or tell me the best way to debug this issue?

Hello there

I have been through similar trouble with workflow

  • Have just created your workflow - attached image and the call was assigned to the opportunity assigned user - worked fine

Only suggestions from me would be

1 - quick repair rebuild

2 - repair relationships