From what I’ve been reading, I think you’re on point. When I multiply or add a DC signal sources with the same sample rate, I know that there are more samples in the FIFO, but it’s still popping. It almost sounds like some of the guys using the USRP had the same problem back in 2013-2014, but they may have resolved it and Nikols Kargas, who made the RFID library, got it to work on the USRP back in 2015.
I suppose it’s time to figure out a gnuradio block to pad in zeros, unless anyone else has any other suggestions or knows of a block that already does this. I’ll also double check that all of the dependencies are fulfilled like @cswiger was suggesting in that OpenBTS post.