Ah, okay - a lighter bullet might just be the ticket. Do the grooves look deep or shallow - conicals do best with shallow grooves in my experience. Go with a bullet that has a fat nose (wide meplate). I would also use a card wad of some sort between powder and bullet, you could make your own from cardboard or similar.

One of the things that encourages full obturation of a conical in a muzzleloader is the weight of the bullet itself. The heavier the bullet, the harder it is to get started and as a result the bullet tends to fill the grooves better. Since you are going on the light side, a bullet with a hollow base might be the best choice but a card wad (or a few grains of buffer such as cornmeal) might well help achieve the good seal you need for accuracy.