Long hair cats and poo-ish question

Sorry for the somewhat gross descriptions that follow...

I have a long hair, neutered, 2 year old male cat. He uses the litterbox fine.

Every now and then, I'll notice him try to scoot on the carpet. When we investigate, there is no stain but there is a foul smell. His butt also smells gross, but there's no poop.

I presume this is an anal sac thing. We saw a vet about a month and a half ago and according to the vet, his anal sacs are not impacted.

I read that the anal sacs express themselves when the cat poops. His poop tends to be quite big, so I don't think it's a matter of the sacs not being stimulated enough to express. 

I have never had a long hair cat before, the others were shothaired and never had this 'no poop smelly butt' issue. 

What I am wondering is whether the sacs are expressing correctly, and it's just getting smeared on his tail somehow?

Also, when he has the scootling and smelly bum, we realized that if we got a wet tissue and wiped his butt and tail (the underside), that cleans off the smell better than using one of those pet wet wipes.

Hi unozero.

Yes, it's very likely that poo is getting stuck to the hair on his behind.  We have a long haired cat also and while it's unusual to happen, sometimes one might get caught and they are really difficult to see.  The smell, is the only clue.  You might think about having your cat being shaved in that area - they do a hygiene cut where  the fur is clipped short enough so nothing gets matted into it.  It can be really difficult to get those out yourself.  You may consider a diet change as well - perhaps he's eating something which is not forming his stool properly.