Chris is 80% right, you are 20% right.
LNB's do differ, but not hugely. the dish makes a lot more difference, but not just the size of it, the alignment of it too, one degree out on the dish alignment and you are pointing about 200 miles away from the sat!
You've not said what size dish you are using or where you are exactly either...most of france is outside the nominal footprint from the astra (sky) sats.
I'd suggest you get a decent installer in and accept you may have to upgrade the dish.