The 'U' problem when sending datas using the USRP

HI!
I am trying to build a flow graph like this :
file_source——>PSK——>usrp sink,there’s no error.
But when I try to execute it,it shows only a U
on the terminal.
According to the answer from this Forum ,it’s because
I’m not feeding the USRP fast enough.So is that the
problem of the file_source’s low speed of reading datas
from my file?
Since I don’t want to change my source ,so can anybody
tell me how to solve the problem?
Thanks a lot!I’m an undergraduate student working on my
graduation thesis,and have great trouble along the way
to finish my own small application on my own.so I really
appreciate your help!