Quen the 6910 comes with 4 baskets.
2 double floored (single and double) and 2 single floored (single and double).
If what I read means you are using the double floored baskets, for a start I dont understand why, if you are grinding your own beans, other than you realise that the Breville grinder is not able to grind fine enough for the single floored baskets.
Assuming (dangerous) that you are aware of this, then you should also know that the grind setting should make no difference whatsoever using a double floored basket.
Fine or course, single or double basket, the pressure is controlled by the one little hole exiting the bottom of the basket.
The only conclusion I can make with the information youve supplied so far is that your double floored double basket is faulty.
What do you mean by "i am not extracting the same flavour "?
How fast is the pour using the single?
How fast is the pour using the double?