[Discuss-gnuradio] Simple HDLC + PSK simulation

Hi All.

Im attempting to create the simplest possible simulation of PSK modulated HDLC Frames. Without modulation, the following works perfectly:

Message Strobe -> HDLC Framer -> HDLC Deframer -> Message Debug.

However when I add a BPSK modulator and receiver between the HDLC blocks. I see nothing printed from the message debug:

Message Strobe -> HDLC Framer -> BPSK Constellation Modulator -> BPSK Constellation Receiver -> HDLC Deframer -> Message Debug.

Strangely, If I place a time sink between the Constellation Receiver and the HDLC defamer, I can see the symbols coming through. I've attached my flow graph and time sink for reference. What am I missing? 

Thank you for your help.