Please don't feel like a failure; all you can do is try to work out what is helping and what not - that is all you can do in this kind of situation once the obvious culprits have been excluded. There is no perfect gut medication for what is a very complex organ.
My 6 year old Dylan has had an increasingly tender digestion so I have taken away his obvious bloating veg; he still has plenty of hay, the veg he can eat, and a little pellets together with extra probiotics and meds whenever the bloating is giving him obvious discomfort. My vet couldn't find anything obviously wrong with him.
I'm not considering myself a failure just because I can't solve a problem that a vet is unable to; especially as we still come up against limits when it comes to health issues with guinea pigs all too often. The horizon has been pushed out a lot over the last 15 years or so but that only brings even nastier and more complex problems within sight.
As a long term owner with the first piggies going back into the 'Stone Age' of half a century ago, the one thing I have learned is that you can only ever do your best at any given time. You are not failing your piggies because you are not superwoman or Almighty God. What counts much more for your piggies is that you give them love and many happy todays while they live because that is how piggies measure a good life by. Concentrate on making their days happy ones filled with enrichment, as much as you can; and you will not fail them.
The piggies of my childhood where not at all unhappy or unloved just because care and veterinary knowledge were rudimentary compared to today. The rest - whether what your piggies pick up or come down with and when and what from they ultimately die is out of your hands anyway.