Problems using USRP as transmitter

Hello All,
I’ve got a simple setup in GRC whereby I record a basic ASK signal
directly to a file from a USRP source. After recording the signal I can
sucessfully play it back to a USRP sink. The problem occurs whenever I
attempt to to skip the first step and playback a pre-recorded signal -
theres no transmission from the USRP. Can anybody explain why this is?
Is there some init done by the USRP source instance that isnt performed
when the USRP sink is created in isolation?


Simon Denvir.