Is the WBS element defined as a required field?
If it is, just another idea... try to insert a 'Press Enter' step right before pasting the value. Just today I had a scenario when I also faced this 'control not found' error after navigating to a screen where I wanted to populate a required field. Since I was certain the control (i.e. the field) name is correct, I was stumped why this doesn't work, but adding the Press Enter step fixed it.