Probably it will be easier to say what tuner you are using then just RTL2832U...
Usually if the scan fails is either because there are no TV channels on that mux or you have poor reception..
So if your tuner is recognized by tvheadend...start with creating a network for your area from the ready made
pre defined muxes...then go back to your adaptor and assign that network..then scan.
If it will fail try the Generic pre defined list (takes longer to scan)but has almost all dvb-t muxes included so you can t go wrong with that.