When I had this problem I found that the TVHeadend preconfigured transponder list were using the wrong transponders for the EPG sources. I manually added in the generally recognised transponders that worked for others using Freesat and after a few hours the EPG populated.
The EPG source which is assigned to each of the channels in your channels list are for the current and next program on that channel alone.
You need the correct EPG transponder for your area.
Shoog