Dried poo constantly stuck to cat's bum

Hi, I know this is common problem, especially for domestic longhairs. We adopted a 2nd cat in October 2013 and ever since about December-January, she ALWAYS has some kind of poo stuck to her bum/around her bum. It seems any time she does number 2 in her well-cleaned litter box, she always gets poop on her butt and it stays there. She does clean herself fairly often and I have seen her try to clean her bum, but clearly she never has success. We tried getting her groomed/shaved but she still has it suck to her bum every other day.

It also smells very bad. After she eliminates (with number 2) if she sits down anywhere, she gets a little fresh poo wherever she sits. Then eventually it dries up and stays on her fur. She absolutely HATES when people touch her tail or attempt to lift it (I've tried multiple times in attempt to clean her bum) she hisses and swipes. (She couldn't get completely shaved because she was fighting with the groomers too much and scratched them a good few times). But if she's walking and her tail is already up I can manage to wipe her bum a bit with this cleansing mousse for cats that I purchased from my vet (it also deodorizes it) and she won't swipe at me then.

This morning, I woke up and she had poo all around where she was sitting and she absolutely REEKED of poo (so strong that I thought for sure she must have defecated somewhere upstairs/outside of her litter box, but I looked everywhere and found nothing - I even looked in her litter box). So now she smells so bad you can't even go near her because it's that gross - it's like that every week.

We've tried different types of litter, moving the litter box, we keep it very clean all the time. To me it seems like a hygiene issue, almost like she doesn't know how to properly use the litter box (if that's even possible?) or how to properly clean herself. Our other cat never has this issue, she's immaculate and NEVER has poo on her butt and never ever smells (she is shorthair). I've spoken to other DL cat owners and some have said this is sometimes a problem, but not a huge deal (in that she smells bad) and others have said that has never been an issue. It's getting to the point where we don't want her on anything, like the couch, which is beige-coloured. 

Anyone with this similar problem or any advice/suggestions on things we haven't already tried? Thanks!!

Oh that stinks, no pun intended. I know when you love someone and you want to be close to them and they stink, well, I'd be sad. And worried.
Actually, maybe the problem is with the food she's eating.
Poop that sticks to fur may be too soft; what is the consistency of her poop?
And smelly poop sometimes means something in the food disagrees with your cat.
Maybe try changing her food: if not grain free, try a grain free food. She may be allergic to a grain (and, grain can be anything from peas to rice to blueberries). Also, proteins such as chicken and fish are common allergens. Better yet, try a novel protein or limited ingredient food, one she's never eaten before, like rabbit or venison. You need only buy a few cans or the smallest dry food bag, a few days worth, to see if her poop starts to smell less offensive.
BTW: I'm assuming she is an indoor cat only. Parasites are always a possibility if she goes outdoors.
PS: SPCA/cat rescue groups/food banks will gladly accept any pet food you can't use, open bags included.

Our cats are medium hair and they have poop butt fairly often. I agree that the smelly poop means trouble digesting food. Our cats have trouble with chicken/poultry (which is hard to avoid in commercial foods) and whenever we feed them products with more chicken the stinky factor goes way up.  When we first noticed the poop problem our vet told us to try cutting specific types of protein for a week at a time to check how they react. Most people say that fish or beef is usually the  problem.  The biggest thing we have done ( other than changing the diet)  is trimming the butt fur.. but obviously that is not going to be easy for you. We also have trouble with them stepping in their poop and tracking it around the house

Thanks guys, I will try some new food maybe! And perhaps a visit to the vet is due regarding this issue.