There are many variables that could affect this - what version of the programs are you
running?
The most common possible causes are
1) the USRP RF/IF gain setting(s) must be tuned for optimum reception. Later versions
of scope.py have a --gains [-N] parameter - you can use the osmocom_fft application to
adjust the gains to find the best values.
2) there is a frequency offset in all USRP's unless you have a GPSDO - the offset
must be accounted for when tuning
3) there is a DC offset (at zero IF) which can be "cured" (circumvented) using
the -o option (I usually use -o 50e3)
4) Trying to receive LSM/CQPSK in disctap mode
For more help we'll need to know the versions of op25 and gr, the command line typed,
and ideally post screen captures of your spectrum, eye, and constellation plots...
Max