You have less control with extract, but there’s still things you can do. What kind of OG/FG are you getting on these beers? Some things you could do:
-Add Maltodextrin. You can add this at any time during the brewing process (8 oz will give you another 4 points or so to your FG on a 5G batch) . For a brunch stout, lactose is another good option (8-16oz per 5G is a good range).
-Some DME brands are more fermentable than others, so you experiment with switching to a different DME brand
-Change yeast. Try using a british yeast, in general they tend to attenuate less than other yeasts. Looks like this kit uses S-04, which should have been a good option.
-Dont add sugar or any other fully fermentable items.
-Steep some extra crystal malt.
