After 30 days, what should I do about my I140 approve notice?

My I140 has been approved pn the website for more than 30 days but I have not received any notice. I called USCIS three times and received three kinds od answer. 1) waite another 30 days; 2) Write a letter to USCIS; 3) Make an appoitment with local office.

Do you have such kind of experience? Is it due to holiday season? Should I keep calling?

Thanks for your suggestion!