My golden has the same kinds of problems. He was diagnosed with yeast infections, put on medication for that, but they never seemed to go away completely.
I was about to begin allergy testing and shots, when it was suggested that I should seek a second opinion. I did, and was surprised when the "new" vet asked if my boy had ever had thyriod function tests. He hadn't, we had the blood test done, it came back with his level being 0.04 he immediately went on medication 2x daily. 30 days later his level was 2.7 - after doing some research on my own I was able to find out that the level suggested is between 2.5 and 3.0.
His skin has cleared up, ear infections went away, he's 100% better.
Of course this may not be the cause for any of your dogs, but if they haven't been tested its something to consider.