I don't get this. Never have. My long haired dogs (border collie, Lhasa) LOVE to be brushed. They'll let me go on forever and boy do they need it. Beagles don't require much in the way of grooming.
But my labs? Flynn loves it but nothing much seems to come out when I use the undercoat comb or even the slicker. Some, but not much. Paddy hates brushing so I just use the palm of my hand on him. That works well. Seamus? Hm, I can get a lick in here and there but again, I don't get much on the brush in return.
So why is it that all the edges of my carpeted rooms, the floor along the underside of the sofas, the base of the kitchen appliances are always practically knee deep in black lab fur?!? Why don't I get that volume with the slicker?
Have you tried a zoom groom or other rubber brush on the Labs?
I found a slicker worked well on Hershey but he had a very thick, almost wooly undercoat. The slicker doesn't do a thing for Casey with her thinner, field lab coat but the zoom groom does grab a lot of loose hair. She seems to tolerate it better, too, although I don't think she'll ever adore being brushed like Hershey did.
I second the Zoom Groom! It seems to get the loose hair off and Daisy doesn't seem to mind it as much as the slicker brush. I usually brush her outside and can get a few rounds in with the zoom groom before she gets twitchy. Then I usually just brush all the extra loose hair off her with my hand. Hopefully the crazy shedding will stop soon... :-\