DPSK losing bytes in simulation

Hi,

I’m very confused with the following flow chart:

file → throttle → dpsk mod —> dpsk demod → unpacked to packed

file sink

I’m losing 2 bytes in the output file.

I’ve uploaded the grc here - www.anfractuosity.com/files/dpsk-test.grc

I realise the data could get corrupted due to the bits not being aligned
to
the byte, however surely it shouldn’t lose 2 bytes for me?

cheers

Chris