No slicker is going to go all the way through a heavy coat. You have to lift each section and work a little at a time. If you just run the slicker over the top of the coat the hair closest to the skin is not getting brushed and will mat
It's easiest to start from the bottom up, sort of like hanging wallpaper in reverse. Start at the dogs foot and pull the hair
upward with one hand and brushing down with the other hand, using your other hand to feed out the next section of coat. For the chest and belly you can use the side of your hand to work the coat. On ears start at the tips of the hair and slowly work your way up, being sure to support the ear with your other hand.