Maybe there is something wrong with the oak barrel or the soil. Try clay pots and new soil.

FataMorgana

Sage needs to drain very well and doesn't like too much water. Top soil might be too heavy, where I live (like Texas) it usually likes rockier, sandier soil. How does it die? Dries out and shrivels up? Gets white all over with powdery mildew and keels over? Bugs or other unwanted guests on the top or under the leaves? (Ask me how I know the many ways it can die...). What I wouldn't give for a few oak barrels!

My sage is in rocky clay soil that's never been amended. It's doing well but not growing as much as sages I have had in the past. I've found with most herbs the best way to grow them is to leave them alone.

I once had some plants in a wooden planter box that were OK the first year. The next year year, they didn't do as well. I couldn't figure it out.
Then, the following year they died. What happened?

I finally realized that the wood that was used to make the flower box had been pre-treated with a preservative - like kerasote or a stain that had something in it.

If that is not your situation, you might consider changing the soil.