There were a few threads a while back where this problem was discussed.
It seemed the issue was never debugged, just disappeared after a clean install.
About as clean as I know to make it:
Running Ubuntu 14.04 LTS from wiped clean (as of 5/4/2015), 64 bit desktop
Downloaded and installed today's Pybombs
Installed osmocom, rtl and gr-fcdproplu
RTL and osmocom work fine.
Completed with a Pybombs update
Rumors else indicate this is a UDEV permissions issue.