Sometimes CIS officer will doubt if you are qualified

to the approved I-140,if you can't find a similar job for a long time, under the situation,the officer may require to review your I-140，other hand,he/she may doubt that you may become a public charge in future.
The immigration officer will think over not only AC21,but also other laws & rules.I ever saw an EB1b
applicant ( I-140 has gotten approval ) met same trouble,his I-485 was tranfered to local CIS to take an interview,& delayed almost two years.