It happened to me as well. I sent FBI certificates for my self & spouse on 23rd Dec and was waitng for PPR , But to my surprise, they had sent a regular mail this week, to submit FBI clearence with in 60 days.
I sent them an email saying that I had already sent them, with all the proofs. They replied me next day, by saying that, they are seeing an update on my file on 31st Jan (after 40 days of receiving documents), so they said, they could be my clearence certificates.
They did not confirm me yet though, but i am hopeful, as I am from last 26 months.
Hopefully, they will update your file in next couple weeks, with your FBI certificates.