Flat G

I purchased a new Conn 8D copy from China. Looks beautiful and craftsmanship is excellent with great sound. However, even with F tuning slide pushed all the way in the middle G in the staff is flat. I try my best to compensate for it but it seems rather excessive. All the other notes are centered except for that G. Anyone know why this may be? Can a new leadpipe fix the problem? The current leadpipe also is slightly wider in diameter at the opening compared to genuine Conn 11D I have. I'm wondering whether changing leadpipe can fix the issue with flat G. Any thoughts would be appreciated.

The medium G is a 5th overtone so it is expected to be 14 flat cents when compared to a tempered intonation. However the notes around G (medium A will be high because they are played with the 1+2 combination in Bb horn the A is even higher because it is a 6th overtone). The medium F#, like the G is also a 5th overtone, but because it is played with the 1+2 combination, it is generally in tune. So, sometimes the G isn't that low, but when compared to the neighbor notes becomes low.
That being said, try the open a, the g# with 2 and the g with 1 using a tuner, to check if these notes are low within the same amount. If that's true the notes will present the expected intonation, however if after doing this test only the G is low, then you've got an intonation issue.
Solutions for this could be to play the g using 1+3 of F0
Different makers build their instruments to adjust these notes, or to make them more flexible.

Actually, the 6th overtone in the harmonic series, our G in the staff is one or two cents sharp compared to even tempered tuning. It is the 5th overtone, our E, that is 13 cents flat. The horn would not come close to playing the harmonic series at all except for the shape of the leadpipe and bell. A hoseaphone without a funnel bell plays the odd harmonics.
I have two horns. On one the G is sharp, and on the other the G is flat. The first branch and bell on the latter horn are misshapen, and I hope that correcting that will help. The horn with the sharp G is German, and that may be on purpose so that the T1 G is not so flat, that being the flat 5th harmonic.

Sorry, wow I see that it referring to the F side, yes that is the 6th overtone, I was assuming it was the middle G on the Bb side as this is usually the problematic one. In that case there is a clear intonation issue

Ah yes, we need to state our assumptions. I assumed that we play our G on the F side, as I learned. Others learn to play it on the B side. I don't like that sound, and I haven't learned to take the "honk" out of it.