sometimes repeated ear infections are caused by yeast, not bacteria.
i have found that if it is a yeasty ear, the antibiotic ear drops will not help at all and sometimes will make it worse.
I use a simple homemade remedy of 1tsp. vinegar to 1/2 cup warm water as an ear wash, it helps to change the ph. of the ear canal to discourage yeast from growing.
If the ear canal is inflammed i will add a couple drops aloe vera juice or lavender essential oil to the mix.
If I think it may be a bacterial issue, I will add either grapefruit seed extract or tea tree oil to the mix.
Hope your dogs ears clear up!