You have a tough cut to be assured of great finished product.I'm certainly no expert on flats.We have cooked a bunch for cater/vend,because less labor.I have attempted more than I like to admit for contests.
I've cooked with and against really good cooks that acquire large[8-10 lb ] C.A.B. flats.Sometimes they can hit great and other times not.Most of them have given up using them for comps.
The small flats that are often available at 3-4 lbs -in non brisket country are typically what the butcher trims out of large pieces.They are good for the nice elderly ladies to pot roast with vegetables until falling apart and then make a nice brown gravy.They make a nice Sunday afternoon dinner and then a nice leftover meal for the hostess lady.My grandmother made some great ones and I still love them.
They seek out the flat that resembles round steak/London broil and has little waste.Even if they are labeled "choice" they are often "bottom" choice.Large old dairy cows can provide the "flat like" cuts for these meals.
Also,depending how many you have cooked and what your feel for "like butter" is,like Jay says"they could be undercooked".
I have been taught that sometimes you might have to take them above 205degrees and then hot box them for 4 hrs and they may rise to 212.
A comp cook often likes large flats,even off their packers,because it gives them more area to select that six to eight slices of better quality to present to the judges.
You usually wind up with the finished quality that you start with.
I am amazed how much better brisket cook I became,when I cooked top quality packers all the time.
If that cut is all you are able to get,there are some techniques to at least help you out.
We have discussed them at length and maybe you could try a couple that way.
Keep trying and you will get better.We all seem to.