Tvheadend

Hi,
I have been using TvHeadEnd for years. I automatically record almost every movie airing on TV based on the TV Guide ratings.
I live in France near Toulouse.

Here we can get the following DVB-T channels (source):

R3: 24H 4kW = Les plages en clair de CANAL+ , C+ SPORT (avec tuner HD MPEG-4) + les chaînes payantes C+ CINEMA et PLANETE+ (TDF sur Lafillaire).
R5: 25H 4kW = TF1 HD , FRANCE 2 HD et M6 HD (TDF sur Lafillaire).
R4: 27H 4kW = M6 SD , W9 , NT1 et ARTE HD + la plage en clair de PARIS PREMIERE (TDF sur Lafillaire).
R8: 32H 2,8kW = 6 TER , RMC DÉCOUVERTE et NUMÉRO 23 (TOWERCAST sur Pech David).
R7: 34H 3kW = HD1, CHÉRIE 25 et L’ÉQUIPE 21 (TOWERCAST sur Pech David).
R6: 36H 4kW = TF1 SD , ARTE SD , TMC et NRJ 12 + la chaîne payante LCI (TOWERCAST sur Lafillaire).
R2: 38H 4kW = D8 , FRANCE 4 , BFM TV , I>TELE , D17 et GULLI (TDF sur Lafillaire).
R1: 55H 4kW = FRANCE 2 SD , FRANCE 3 TOULOUSE , FRANCE 5 , LCP et FRANCE Ô (TDF sur Lafillaire).

My reception should work fine, since my Sony TV can find all these channels when plugged directly.
During a long time, I have been missing 6 channels from 2 multiplexes: R7 and R8.
My TV card id a dual tuner : Hauppauge WinTV Nova-T-500 which works very well, but only finds multiplexes R1 to R6.

Last week, I bought a new tuner (AverTV Volar HD 2 DVB-T USB stick). It was not supported under OpenElec, but thanks to the community, it now works fine.
Using this new tuner, I was able to find multiplex R8 and its 3 channels. I cannot figure out why this tuner is able to find this multiplex and the Hauppauge isn't.

I still miss multiplex R7. I have tried to find it automatically and also manually, without any success. It should be on channel 34 = 578 MHz (source).
Do you have any idea?
Thank you very much.

I use TvHeadend 4.1-1242 under OpenElec 6.0.1.