That all depends on the quality of dogs you're working with ... your understanding of what's behind them ... and the depth of your belief in them.
Why isn't it working?
Is it because of the lack of quality originally in the blood, the lack of prepotency in the individuals you've selected, what?
If you make the statement "it's not working" ... then the nugget you have to dig out is WHY is it not working?
I am not being mean, but the missing qualities might not be in the dogs but in your selection of which dogs to use. On the other hand, if the blood really isn't any good then there is no reason to fool with it at all. IMO, if the blood cannot stand on its own merit then what do you need it for?
However, you must also understand that World Class ability can skip a generation or two ... as long as the dogs are at least game and competitive, you still have something good. Just re-shuffle the deck and you can get badass winners again. Case in point, look at the pedigree of Gr Ch Junior 6xW. His parents were mediocre, game dogs. His grandparents were all losers ... but behind that there are more 6x & 7x winners
Point being that these 6-7x winners sired and whelped a bunch of losers. These losers were interbred some more to get some fairly-decent dogs, and then these were interbred again to get Junior ... who was a 6xW. Breeding success is about ebbs and flows, and you can't realistically expect to get Grand Champions in every litter. Your dogs will revert back to average ... but if you stick with it, and re-shuffle the deck correctly, you can get exceptional dogs again.
I am being realistic, and yes you can do it forever ... IF you're dedicated enough, IF you have truly good stock to begin with, and IF you manage your genetic pools correctly. Another case in point: Northern Express' Ch Red Bull. This dog's mama (Perfect) was a bum. Her daddy Phoenix was a bum. Yet the dog Red Bull (and every dog in his litter) were B+ to A caliber dogs. How did this happen?
Well, I produced Red Bull by breeding an excellent, highly-inbred dog to a bum, highly-inbred bitch. If you click on the pedigree and study it as you read ... to the story of the bum mama started when I bred Poncho to his sister Missy in 1997 and got Phoenix, a game bum. Meanwhile, in 1999 I bred Poncho to his daughter Screamer (a flawless, topshelf bitch) and got Jezebel, an ace. Well, I bred Phoenix to Jezebel in 2001 and I got 2 real good males (Stone Cold and Zephyr) and the cold idiot-bitch Perfect. That's the bottomside of Ch Red Bull.
To make the topside of Ch Red Bull, back in 1994 Mr. Nice Guy bred Poncho's excellent sister Ruby to Ch Commanche and got Ch Tank 4xW, Roy Jones Jr. (an excellent dog), and Rocio ROM. The breeder then bred his pick, Roy Jones, back to his mama Ruby to get my inbred Tuffy bitch, who was rough as hell but short-winded. Meanwhile, as previously-stated, the inbred ace Jezebel (above) had an average, but exceptionally-longwinded brother named Duke Nukem (Poncho/Screamer), who I bred to Tuffy to get another flawless specimen named U-Nhan-Rha, who was a VERY solid, capable animal.
Well, when I bred the flawless U-Nhan-Rha to Perfect, even though Perfect was herself a bum, she was rugged and durable, and her mama was an ace and was Duke Nukem's sister, so I was doubling-up on Duke Nukem and Jezebel ... and the result was an all-excellent, match-quality litter, and one of the tightest breedings I ever made in my life, actually using a bum as the mama.
Yet Red Bull and his littermates were all badass dogs ... off of nothing but a continuous string of brother/sister, father/daughter, mother/son, cousin-cousin inbreedings!
So, if you really believe in your stuff, and if you understand the "ebbs and flows" of ability ... so long as your stuff remains game ... you can continuously "re-shuffle the deck" with your inbred dogs and get the ability back again ... if you play your cards right
Hope this helps,
Jack