Multiple minor health issues

I have three cats - Amelie, 1 year and a half, is in perfect health. The other two, however, keep having minor health issues and I am wondering what I am doing wrong.

Harley is Amelie's brother, 1 year and a half, neutered. When I found him on the street he had an aggressive herpes virus and he has a permanent damage in his right eye. Soon after I took him indoors he had fever and was vomiting a lot (this was more than a year ago). All those antibiotics left him with chronic diarrhea. After many trials and errors with different diets and meds, his digestive issues are gone.

But NOW, after months of being in perfect health, he developed some tiny little crusts on his neck. The vet said it is dermatitis, most likely food related, and, of course, to put him on hypoallergenic dry food (which is a no-no as I do not feed dry food, but I am willing to try an elimination diet).

Sophie is 2 years and three months old, poor immune system because of the ringworm she had when she was born. During these years, the most serious health problem was a constipation and some sort of colitis that made her poo with blood. That is also solved, thanks to SEB and a change in her diet. But then her ears started to get dirty - no ear mites, just excessive wax and we have to take her to the vet every two months for a thorough cleaning. 

But NOW she also has gingivitis. Early stages. She had her teeth cleaned and we have a spray to clean her teeth with.

So I know the treatments and I have a good relationship with my vets, but no one is able to tell me WHY my cats have these problems. What am I doing wrong? While I fed dry food, Harley had chronic diarrhea and Sophie had severe constipation. Now, on 70% wet and 30% raw, Harley has skin issues and Sophie has some sort of otitis and gingivitis.

Sometimes there is NO answer, reason. You're doing everything right, and things still happen.
I fed prey model raw (with bone), and Ritz still has to have her teeth cleaned about every other year. I chalk it up to her poor diet when she was young (abandoned/dumped; fed some milk and cereal until trapped).
I would try the elimination diet for the skin condition. And I'd read/study the labels from the dry food/wet food and see what is the same in both (additive? protein). Can be miniscule, but enough for cats who are very sensitive.

It could be a protein, it could be something in the premix, it could be anything else in the 70% canned.  I feel for you, it's awful trying to get some kitties to eat healthy and eliminate what's bothering them.

Nothing you are doing wrong!

For Sophie, I would start brushing her teeth. I brush my cat's teeth at least 3 times a week and he STILL needs dentals but I like to think that the brushing helps keep the dentals at every 2 years rather than yearly. I would also get the solution and a cotton ball and clean her ears weekly. Ask your vet to show you how. Maybe you will need to bring her in less often for a thorough cleaning if you did it weekly.

Also...I didn't start brushing Malibu's teeth until he was 5. I put him in a Cat Sack and he tolerates it very well...well he might growl at me sometimes if he's not in the mood but he never bites

Thank you for your replies.

The vet gave as a spray for Sophie's gums and some antibiotic for Harley. If he has allergies, can an antibiotic help?

Maybe the spray for her gums. I haven't heard of antibiotics helping allergies---you're sure it wasn't some type of allergy medication?